A new method for an assay of human granulocyte colony-stimulating factor (hG-CSF) has been developed using human promyelocytic HL-60 cells. The proliferation of HL-60 cells had been suppressed by the addition of dimethyl sulfoxide (DMSO) or retinoic acid (RA). These differentiating agent-treated HL-60 cells exhibited an increase in their number in response to recombinant hG-CSF (rhG-CSF). Neither dibutyl-cAMP nor interferon-γ (IFN-γ)-treated HL-60 cells, however, showed an increase in their number in response to rhG-CSF. The proliferation rate of DMSO-pretreated HL-60 cells was linearly increased from 0.3 to 10 ng/ml of rhG-CSF. L-Value of HL-60 cells assay was 0.027±0.012. The activities of non-glycosylated rhG-CSF produced by Escherichia coli and glycosylated rhG-CSF produced by chinese hamster ovary (CHO) cells were compared using DMSO-treated HL-60 cells; no significant difference between them. DMSO-treated HL-60 cells also responded to interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF), but did not respond to erythropoietin or macrophage colony-stimulating factor, suggesting that the responsiveness of these cells to growth factor is restricted to myelogenic cytokines. In conclusion, DMSO- or RA-treated HL-60 cells are useful for the measurement of bioactivity of hG-CSF.
To develop the immunochemical methods for determining 1α, 25-dihydroxyvitamin D3 [1, 25(OH)2D3] in clinical samples, a variety of monoclonal anti-1, 25(OH)2D3 antibodies have been generated. Two kinds of hapten-carrier conjugates, 25-hydroxyvitamin D3 3-hemisuccinate (hapten 3-HS) and 1α-hydroxy-25, 26, 27-trinorvitamin D3 24-oic acid (hapten 24-OA) conjugated with bovine serum albumin, were used for immunization. Spleen cells from SD rats or BALB/c mice, each immunized with the conjugate of hapten 3-HS or 24-OA, were fused with P3/NS1/1-Ag4-1 myeloma cells. After screening by ELISA employing β-galactosidase-labeled haptens, seven kinds of hybridomas secreting anti-1, 25(OH)2D3 antibodies were established. Binding characteristics of these antibodies (Ka 0.73-20×109 M-1) were investigated by an RIA using tritium-labeled 1, 25(OH)2D3. The data suggested that the rat monoclonal antibody 3R-1 derived from the hapten 3-HS and the mouse monoclonal antibody 24M-3 from the hapten 24-OA would be available for developing practical analytical systems.
The effects of Hange-shashin-to (TJ-14) were examined regarding the amount of prostaglandin E2 (PGE2) and the water absorbind capacity in the large intestine of rats. Repeated oral administration of TJ-14 at doses of 125 to 1000 mg/kg revealed a significant decrease in PGE2 content in the colonic mucosa and also the promotion of colonic water absorption in a dose dependent manner. However, there was no remarkable influence on the concentrations of aldosterone and electrolytes in the serum, even at 1000 mg/kg. From these results, it was considered that some of the anti-diarrheal effects of TJ-14 might be based on a repression of the increase in the amount of PGE2 as well as promotion of the water absorbing capacity of the large intestine. Moreover, it was also suggested that it is possible, by the application of TJ-14, to prevent the loss of water content caused by diarrhea.
The present study investigated the selective cytotoxicity of piperine on cultured brain cells by using lactate dehydrogenase release, 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium reduction, and protein content assay. Exposure to piperine induced marked injuries on cultured hippocampal neurons whereas it induced only a marginal toxicity on cultured astrocytes. The mechanism of cytotoxicity in hippocampal neurons was studied by pharmacological methods. The membrane-permeable free radical scavengers, D-α-tocopherol and trolox, protected neurons from piperine toxicity whereas membrane-impermeable agents, superoxide dismutase and catalase, were ineffective. A lipoxygenase inhibitor, nordihydroguaiaretic acid, but not a cyclo-oxygenase inhibitor, indomethacin, attenuated piperine toxicity. We provide the first evidence that piperine induces selective neurotoxicity in vitro. Although the exact mechanism is still unclear, pharmacological evidence indicates the possible involvement of lipid peroxidation and the generation of free radicals, at least partly, through an arachidonate-lipoxygenase pathway.
Changes in the mRNA levels for aldosterone synthase cytochrome P450 (CYP11B2 or P450aldo) and 11β hydroxylase cytochrome P450 (CYP11B1 or P45011β) in rat adrenal glands were studied in response to angiotensin II type 1 (AT1) and type 2 (AT2) receptor antagonists. CYP11B1 and CYP11B2 genes were highly homologous (88.5%) in their nucleotide sequences of the amino acid coding regions. Reverse transcription-polymerase chain reactions (RT-PCR) which are capable of discriminating between rat CYP11B1 and CYP11B2, were performed with specific primers for each P450. Upon sodium restriction (5 mmol Na+/kg of diet) of rats for 14 d, the amount of the CYP11B2 mRNA in the adrenal glands was increased 8.5-fold compared to that from the rats fed a normal diet (225 mmol Na+/kg of diet), whereas no significant change in the CYP11B1 mRNA was observed after the dietary sodium restriction. As shown by an immunoblot analysis, the adrenal capsule portions (mainly zona glomerulosa) of the rats kept on the low Na diet for 14 d expressed significantly higher levels of both CYP11B2 and CYP11B1, and contained a significantly higher amount of CYP11B2 than those from the rats fed by normal diet. The activities of the CYP11B2 enzyme were also found to be increased by about 8-fold on day 14. In concert with these alterations, the plasma aldosterone concentration (PAC) increased. However, when the specific AT1 antagonist E4177 was given to rats maintained on the low Na diet, the amount and activity of CYP11B2, as well as the PAC, were suppressed. In contrast, the increase in CYP11B2 induced by the low Na diet was not affected by the AT2-specific antagonist PD123177. These results indicate that the aldosterone synthase cytochrome P450 (CYP11B2) is an ultimate target of the regulation of aldosterone biosynthesis by an AT1 receptor antagonist.
Disposition of 3, 4-methylenedioxymethamphetamine (MDMA) in hair roots was studied using rats and the hair root samples were evaluated to prove acute MDMA poisoning. The back hair of male pigmented hairy rats (n=6) was shave and 5 d later the animals were intraperitoneally administered with acute poisonous doses (20, 40, 60, 80 and 100 mg/kg) of MDMA. Roots of the hairs were then plucked out with a hair nipper 5 min and, 0.5, 1, 2, 6 and 24 h after injection. The hair root samples were, directly or after being washed with detergent, extracted with methanol-5 N HCl (20 : 1) under ultrasonication in ice-cold water for 4 h. After filtration and evaporation, the residue was derivatized with pentafluoropropionic anhydride and analyzed by GC/MS. From all samples including the 5 min sample, MDMA was detected at high concentrations (up to 156 ng/mg) accompanied by 3, 4-methylenedioxyamphetamine (MDA). Some of the animals died within 2 h after administration, but in the surviving rats the MDMA concentrations in the hair roots increased up to 6 h and then slowly decreased until 24 h. The remaining MDMA after washing apparently increased from 13-31% at 0.5 h to 51-83% at 24 h in the surviving rats. These facts show that most of drugs in the hair roots are not yet immobilized in the early stage and are thereafter gradually incorporated into the hair shaft. Increase of the MDMA concentration stopped soon after death of the animal, probably due to the cessation of hair growth. Although the ratios of MDA/MDMA steadily increased over time, those after death plateaued, probably due to the cessation of metabolism after death. It was clearly shown that MDMA is more quickly incorporated into and more firmly retained in hair than methamphetamine (MA) by comparing their disposition in hair roots.
The inhibitory effects of several anti-inflammatory agents, including glycyrrhizin (GL), on the activities of hyaluronidases (HAses) purified from bovine testes and Streptomyces were investigated in vitro. It was found that (i) GL inhibits the activity of HAse (p55) from bovine testes in a dose-dependent manner, but does not affect HAse from Streptomyces; (ii) GL was the most effective of the compounds tested on bovine testis HAse activity (50% inhibition with approx. 3 μM GL); and (iii) glycyrrhetinic acid (GA), a derivative (oGA) of GA and diglucuronic acid had no detectable effects on HAse activity at 9.0 μM. The GL-induced inhibition of HAse activity is uncompetitive for its substrates. Data are provided to support the contentions that (i) bovine testis HAse (p55) is a GL-binding protein; and (ii) GL acts as a potent inhibitor of HAse in vitro.
Nine alkaloid constituents in the root of Aconitum yesoense var. macroyesoense, as well as three acetylated derivatives, were examined for their peripheral vaso-activities by measuring laser-flowmetrically the cutaneous blood flow in the hind foot of mice after intravenous administration. The major constitutive delcosine (1), 14-acetyldelcosine (2) and lucidusculine (3), respectively, had little or very mild vaso-activity. Kobusine (4) and pseudokobusine (5) and three minor constituents, luciculine (6), 1-acetylluciculine (7) and dehydroluciculine (8), together exhibited a rapid increase in blood flow reaching a peak with a magnitude almost equal to that produced by hydralazine, when administered intravenously at the same dosage level of 20 mg/kg. Among them, 4 was characterized by successive reversal of the increase to a decrease in blood flow, while 7 produced a flow with a more delayed peak time. Dehydrolucidusculine (9) exhibited a transient decrease in blood flow prior to occurrence of the increase, as did papaverine. Consequently, it is assumed that the alkaloids, especially those of the C20-diterpenoid type, in the root of this Aconitum plant have peripherally vaso-dilating activities to varying degrees in mice, probably due to their direct action on the cutaneous microvasculature in a similar fashion to that shown by hydralazine. The laser blood flowmetric method would be useful as an in vivo means of qualitative as well as quantitative screening of chemically modified derivatives of peripherally vasoactive agents in mice.
To investigate the permeability of natural compounds through hairless mouse skin, compounds having a range of lipophilicity, i.e., ginsenoside-Re (1), baicalin (2), glycyrrhizin (3), baicalein (4), wogonin (5), honokiol (6), magnolol (7), bergapten (8), shikonin (9) and sinomenine (10) were used. These compounds permeated through the skin a little, however, they were generally accumulated into the skin. The uptake amount into the skin of each compound related to their lipophilicities in the in vitro experiment. Furthermore, Ligustici Chuanxiong Rhizoma (Senkyu) ether extract (SEE) enhanced their permeability into the skin; especially, it exhibited an effect on the skin permeability of moderately lipophilic compounds such as 4, 8. The effect of SEE in vivo was similar to that obtained in the in vitro experiment. From these results, it was clarified that natural compounds having high lipophilicity sufficiently permeated into the hairless mouse skin owing to their accumulative property, and SEE enhanced the permeability of the moderately lipophilic compounds into the skin.
The preventive effects of saponins from Puerariae Radix toward in vitro immunological liver injury using an antiserum against the rat liver plasma membranes on primary cultured rat hepatocytes were studied. Crude saponin from Puerariae Radix inhibited the elevation of alanine aminotransferase (ALT) activity at the dose of 90 μg/ml. The inhibition was stronger than that of glycyrrhizin, which was a positive control drug. The representative saponins in this drug, soyasaponin I and kudzusaponin SA3, were also more effective than glycyrrhizin, although their effects were weaker than that of crude saponin at the lower doses (90, 200 μg/ml). At 500 μg/ml, kudzusaponin SA3 showed antihepatotoxic activity equal to that of crude saponin.
We studied the plasma pharmacokinetics of the lactone form and the carboxylate form of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan (CPT-11), after intravenous bolus administrations of each form of SN-38 and of CPT-11 to rats. After the SN-38 lactone injection, SN-38 lactone was predominant at first, and then the lactone to the carboxylate concentration ratio (LC ratio) was maintained from 30 to 90 min after the injection. The carboxylate was predominant throughout the period after the carboxylate dosing. Model-dependent analyses showed that the SN-38 lactone had greater plasma clearance and a greater distribution volume than the carboxylate. The CPT-11 administration resulted in a predominant plasma SN-38 lactone concentration. The contribution of the SN-38 lactone AUC to the total SN-38 AUC (57%) was independent of the dose of CPT-11. These results suggest that it is possible to estimate the SN-38 lactone concentration and AUC from the total SN-38 concentration without separate determination of the lactone and carboxylate. Our results showed that both SN-38 lactone and CPT-11 administration gave the predominant SN-38 lactone in plasma; hawever, only CPT-11 could sustain the lactone concentration at a high level, which is necessary for antitumor activity.
The effects of buprenorphine (BNP, 10-200 μg/kg, i.v.) and pentazocine (PTZ, 2.5-10 mg/kg, i.v.) on the regional cerebral metabolic rate for glucose (rCMRglc) were analyzed in nine anatomically discrete areas of the conscious rat brain by the simultaneous use of [14C]2-deoxyglucose, the glucose analogue that can be phosphorylated in the brain, and [3H]3-O-methylglucose, a nonmetabolizable glucose analogue. Originally, this method was developed by Gjedde and Diemer in the rat and in humans. The rCMRglc was significantly decreased by BNP (100 or 200 μg/kg) in most of the brain regions investigated, except the cerebellum. In contrast, PTZ (10 mg/kg) significantly increased rCMRglc in the cerebral cortex and medulla. In the cerebral cortex and medulla, the direction of the effect on rCMRglc was opposite for BNP (22% decrease at the dose of 200 μg/kg) and PTZ (22% increase at the dose of 10 mg/kg). These findings strongly suggest that the discrepancies between the marked effects of BNP (a partial μ agonist and κ antagonist) and PTZ (a μ antagonist and κ agonist) on rCMRglc reflect the selectivity of agonist action at the different types of opioid receptors, μ and κ receptors, in the rat brain.
Single-strand breaks (ssb) in double-strand (ds) DNA produced by hydroxyl radicals (·OH) generated by Cu(II) complexes of podophyllotoxin (PD)-related compounds were evaluated using S1 nuclease digestion. Cu(II) complexes of VP-16 (etoposide, 4'-demethylepipodophyllotoxin-9-(4, 6-O-ethylidene-β-D-glucopyranoside)), 4'-demethylepi-PD (DEPD), and syringic acid (SA) exhibited both ssb and ds breaks (dsb) in ColE1-HaeII and pBR322-BglI DNA fragments, in which the number of ssb was found to be more than three times and four times greater than that of dsb, respectively. Cytosine (C)methylation of cytosine-guanine doublet (CpG) in pBR322-BglI DNA inhibited both ssb and dsb within DNA segments by ·OH generated by the Cu(II) complexes.
Fungal β-glucans have abilities to induce NO (nitric oxide) synthesis by macrophages in vivo, and the intensity of NO synthesis significantly varied dependent on the structure of β-glucans. The molecular mechanism of NO synthesis by β-glucans, however, has not been clarified in detail. To determine β-glucan-mediated NO production, we used various β-glucans (SPG-OH, GRN; Grifolan, SSG, OL-2, ZYM; zymosan A and ZYC; zymocel), which could enhance NO synthesis in vivo, and stimulated peritoneal macrophages (PMs) in vitro in the presence of interferon-γ (IEN-γ). Using recombinant cytokines, a minimum concentration of the cytokines for NO induction was about 20 ng/ml in the presence of IFN-γ under the experimental conditions. Of β-glucans tested, only SPG-OH and GRN produced high concentrations of IL-6 in the culture supernatants. SSG also induced NO synthesis in vitro, but concentrations of inflammatory cytokines were low even in the presence of IFN-γ. In addition, there are some β-glucans which could induce NO synthesis in vivo but not in vitro (OL-2, ZYM, ZYC). These findings suggested that NO productivity of β-glucans in vivo is regulated by several mechanisms.
At present, most Neisseria gonorrhoeae testing is done with β-lactamase and agar dilution tests using common therapeutic agents. Generally, in bacteriological diagnosis laboratories in Argentina, study of antibiotic susceptibility of N. gonorrhoeae is based on β-lactamase determination and agar dilution method using common therapeutic agents. The National Committee for Clinical Laboratory Standards (NCCLS) recently described a disk diffusion test that produces results similar to the reference agar dilution method for antibiotic susceptibility of N. gonorrhoeae. We obtained 57 gonococcal isolates from patients attending a clinic for sexually transmitted diseases in Tucuman, Argentina. Antibiotic susceptibility tests using agar dilution and disk diffusion techniques were compared. The established NCCLS interpretive criteria for both susceptibility methods appeared to be applicable to domestic gonococcal strains. The correlation between the minimum inhibitory concentration (MIC's) and the zones of inhibition was studied for penicillin, ampicillin, cefoxitin, spectinomycin, cefotaxime, cephaloridine, cephalexin, tetracycline, norfloxacin and kanamycin. Dispersion diagrams showed a high correlation between both methods, with a sensitivity of 89% and specificity of 91%.
Reverse transcription (RT) competitive polymerase chain reaction (PCR) is a sensitive and useful technique for the absolute quantitation of mRNA level. We developed a competitive PCR method with a compact digital camera DC 40 (Eastman Kodak Company, NY, U.S.A.) for the measurement of human β-actin cDNA (mRNA), a reference gene in an analysis for target gene expression. The devised method, compared with currently used methods, was easy, safe, sensitive, and highly reproducible to assay an infinitesimal level of cDNA (mRNA).
Gallic acid (GA) and chebulagic acid (CA) were isolated from the extract of a herbal medicine, kashi (myrobalans : the fruit of Terminalia chebula) as active principles that blocked the cytotoxic T lymphocyte (CTL)-mediated cytotoxicity. GA and CA inhibited the killing activity of CD8+ CTL clone at IC50 values of 30 μM and 50 μM, respectively. Granule exocytosis in response to anti-CD3 stimulation was also blocked by GA and CA at the equivalent concentrations.
In order to clarify the arrhythmogenic effects of nonsedating antihistamines, we examined the effects of astemizole, a nonsedating antihistamine, on ventricular activation and RT intervals in a canine myocardial infarction model. Myocardial infarction was produced by two-stage ligation of the left anterior descending coronary artery in dogs. Seven days after ligation, bipolar electrodes were sutured on the ventricular surface of the infarcted and normal zones to apply an electrical stimulation or record the ventricular activation. An electrical stimulation with coupling intervals between 300 and 140 ms was applied on the ventricular surface of the normal zone during atrial pacing, and the ventricular activation delay was measured. The effect of astemizole on the RT interval was also determined during atrial pacing, sinus rhythm or after premature stimulation. The ventricular activation delay increased after astemizole at doses of 0.3 to 3 mg/kg in the infarcted and at 3 mg/kg in the normal zones, and the effect of astemizole was greater in the infarcted zone. Astemizole increased the RT interval in the normal zone to a greater extent at a long coupling interval. The increase in the RT interval was greater in the infarcted zone compared with that in the normal zone. In conclusion, astemizole increased the activation delay in the infarcted zone, probably through prolongation of the repolarization time, and its effect on the activation delay may be arrhythmogenic.
The intestinal absorption of oligopeptides, peptidase-degradable tyrosylglycylglycine (TGG) and peptidase-resistant L-tyrosyl-D-arginine (D-kyotorphin, D-KTP) across Peyer's patches (PP) in rabbit intestine were studied using an Ussing-type chamber and in situ closed perfusion methods. The clearance for the serosal appearance of intact TGG across PP by the Ussing-type chamber method was a little higher than that across the jejunal epithelium (JE). Meanwhile, the in situ closed perfusion experiment showed that the clearance for the plasma appearance of intact TGG across PP was about 10 times that of JE. Furthermore, it was shown that the clearance for the plasma appearance of D-KTP in PP was about twice that in JE by the in situ closed perfusion method, indicating that the membrane permeability of PP was higher than that of JE. Therefore, these results indicate that PP had less metabolic peptidase activity than JE, and that the PP was a suitable site for the intestinal absorption of oligopeptides, especially peptidase-degradable peptides.
We previously reported that the pharmacokinetics of cyclosporin A (CyA), particularly absorption, were altered in diabetic rats treated with streptozotocin. In the present study, the effect of diabetes on pharmacokinetics of CyA after intravenous and oral administration of CyA using the blood and lymph and the gastrointestinal transit were examined in Goto-Kakizaki (GK) rats, a genetic model of non-obese non-insulin-dependent diabetes mellitus (NIDDM), and compared to non-diabetic Wistar rats. Although the systemic and lymphatic availability after intravenous administration of CyA to the GK rats was not significantly different from those of the control Wistar rats, those availability after oral administration of CyA to GK rats was markedly reduced in comparison. These results suggest that the pharmacokinetics of CyA, particularly absorption, was altered in GK rats. Studies on the gastrointestinal transit in GK rats showed that the gastric emptying rate was lower than that of Wistar rats, suggesting that a change in gastrointestinal transit in GK rats may influence the absorption of CyA. The gastric emptying rate in GK rats altered not only the systemic availability but also the lymphatic availability, suggesting that the altered systemic availability may cause adverse effects and that altered lymphatic availability may influence the immunosuppressive effects.
To clarify the dominant mechanism for the convulsant activity of H2 antagonists, the effects of an H2 antagonist, cimetidine, on membrane currents induced by various agonists were investigated. In Xenopus oocytes injected with mouse-brain mRNA, acetylcholine (ACh), serotonin (5-HT), γ-aminobutyric acid (GABA), glycine (Gly), glutamic acid (Glu), kainic acid (KA), quisqualic acid (QA) and N-methyl-D-aspartic acid (NMDA)-induced current responses were recorede under a voltage-clamp condition. Cimetidine inhibited GABA-induced currents in a concentration-dependent manner; however, the current responses induced by the other agonists were not modified. The IC50 of various H2 antagonists, famotidine, nizatidine, cimetidine and ranitidine, for GABA (10 μM)-induced current response were 66, 260, 450 and 980 μM, respectively. However, these values of cimetidine and ranitidine were 40-400 times higher than the reported brain and cerebrospinal fluid (CSF) concentration of H2 antagonists at the occurrence of a clonic convulsion in vivo. In conclusion, we observed an inhibitory effect of H2 antagonists on the GABA response; however, this inhibition of GABA-mediated neurotransmission may not be the dominant mechanism for H2 antagonist-induced clonic convulsion in vivo.
The present research was set up to verify whether the chemical products used in lemon production (from cultivation until packaging) have a bactericidal or a bacteriostatic ability against Vibrio cholerae O1. The studied products were : copper oxychloride, benomil (a carbamate), active chlorine, sodium-o-phenylphenoate, guazatine (a polyamine mixture), imazalil (an imidazole) and lemon peel. The latter was studied with and without treatment using the above mentioned chemicals. Different dilutions of these products were tried out with varying exposure times against the bacterium V. cholerae Serogroup O1, Biotype El Tor, Serotype Inaba. The concentrations of the microorganism ranged from 102 to 108 CFU ml-1, the latter one being considered an infectious dose. The following results were obtained : 1) active chlorine (chlorinated water) showed bactericidal activity at concentrations of 50, 100 and 200 ppm after 10 min of exposure time, 2) copper oxychloride, sodium-o-phenylphenoate, guazatine and imazalil showed bactericidal activity against V. cholerae at concentrations of 102 and 104 CFU ml-1, 3) due to the fact that during its cultivation the fruit is successively sprayed with several chemical products, it could be that the result of the successive treatments is superior to the result of a repeated treatment with each of the individual products. This consideration should be taken into account when evaluating the eventual protection of the lemon.
We previously reported that Bordetella calmodulin-like protein (CLP), like bovine brain calmodulin (CaM), enhances adenylate cyclase and phosphodiesterase activities. In this communication, antigenic and biochemical characters were compared between CLP and CaM. It was found that anti-CLP and anti-CaM sera obtained reacted with CaM and CLP, respectively. The amino acid composition of CLP was similar to CaM. The similarity was also found in an Escherichia coli acyl-carrier protein (ACP), a CA2+-binding protein. Though the amino acid sequence of N-terminus region of CLP has no significant homology with CaM, ACP has highly homology in its N-terminus similar to CaM. These results suggest that CLP might act as a Ca2+-binding protein, and/or an ACP during the activation of adenylate cyclase and phosphodiesterase.
Protein kinases from rice embryos and leaves were identified, and the manner in which they are affected by plant hormones and light was studied. Ca2+-dependent protein kinase activity in embryos increased in early seedlings, but was inhibited during incubation in the presence of abscisic acid for 2 d. Ca2+-dependent protein kinase activity in leaves increased in the presence of gibberelline for 7 d. Ca2+- and phosphatidylserine (PS)-dependent protein kinase activity was found primarily in the cytosol fraction of rice leaves. In the dark, this Ca2+- and PS-dependent protein kinase activity could be detected in the membrane fraction of etiolated shoots. Protein kinases appear likely to regulate plant growth through protein phosphorylation.
3β, 5α-Dihydroxycholestan-6-one was detected in fresh human plasma at the concentration of 14-44 ng/mL. The oxysterol binds specifically to phorbol ester specific binding protein in vitro, and may be an endogenous ligand of the protein.