水銀中毒に関する研究 : I. リポ酸の解毒作用について : 第2報 実験的水銀中毒に及ぼすLipoic acidの解毒効果

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Part 1. The reaction with mercuric ions in vitro The marked effectiveness of BAL (2, 3-dimercapto-propanol) against arsenic and mercury poisoning has been well recognized. The recently identified new vitamin "α-Lipoic Acid (6, 8-dithiooctanoic acid)" has a disulfide ring and is readily reduced in vivo by DPN enzyme to dithiol compound resembling chemically to BAL. The possibility of combining with mercury is discussed elsewhere, but no experimental results have been published here-to-fore. In the present report, Lipoic Acid (LiA) and dihydro-LiA were mixed with HgCl2 or phenyl-mercuric acetate (PMA) solutions (each in 6mM) and precipitations there formed were centrifuged at 10, 000 rpm., and Hg and dithiol color reactions were tested with supernatant liquid and sediment. In order to obtain quantitative informations mercuric compound above mentioned were labelled with Hg203 and used in the second experiment. The possibility of combining of Hg ions with carboxyl radicals could not be excluded, so each precipitate was tested regarding the reaction with pH and the reducing agent. With the purpose to block the carboxyl, a derivative "lipoamide" was also used in this experiment. The results obtained are summarized as follows: 1) Inorganic mercuric ions reacted with dihydro-LiA forming the six membered ring in equivalent molar concentration. 2) This reactant was insoluble in mineral acid or organic solvent, but readily soluble in strong alkaline reaction (pH 11∼12). 3) This solution showed a strong absorption at 350mμ, shifted to the longer wave length by 20mμ compared with original LiA. This fact is to be regarded to support strongly the hypothesis of ring formation of Hg with dithiols. 4) Organic mercuric compound was found to react only with one SH, thus enabling dihydro-LiA to combine with twice as much molar organic mercury. 5) LiA which has no free SH was also able to combine in irregular manners with Hg ion, even the lipoamide could trap slightly the ion. These reactants were readily sedimentated at pH 4 or below it. These phenomena were to be regarded as suggesting the possibility of combination of Hg with carboxyl or S-S bond thus forming hardly dissociable compounds which are liable to percipitate. Part II. Antidotal Effect of Lipoic Acid against Experimental Mercury Poisoning in Rats and Mice The experiment that the recently identified vitamin "Lipoic acid" and especially its reduced form "dihydro-LiA" reacted with Hg ion in vitro and formed an insoluble compound was presented in Part I of this paper in details. In the present experiments, the evaluation of its antidotal capacity in vivo was investigated. 1) Toxicity of Hg for acute poisoning was first tested in mice. LD50 for HgCl2 was 27 mg/kg (subcut.), for phenylmercuric acetate it was 35 mg/kg(subcut.). 2) To determine the maximum therapeutic dose, LD 50 of antidotes were also determined: LiA 264 mg/kg (intraperit.), dihydro-LiA 184 mg/kg (intramus.), and BAL 128 mg/kg (intramus.). 8) Effects of LiA and BAL against severe poisoning in mice. LAi (55 mg/kg) was given by intraperitonea route simultaneously with HgCl2 (intram.), and four equal doses of LiA were given intramus cularly after 4 hours and on the following three days. Equivalent amount of BAL were used in positive control-group. HgCl2 was given in graded doses from 20 to 200 mg/kg as shown in the Table 5, The antidotal effects were summarized in Table 6. LiA increased LD 50 by 1.7 times, BAL by 5.4 times. Accordingly, BAL was triply more effective than LiA. Dihydro-LiA was not tested because enough quantities was not available, but it was supposed to be more effective than LiA against acute poisoning. 4) Effects on the distribution and excretion of Hg203 in rats. 6 mg/kg of PAM (Hg203 1.2-2.5μc) was given intramuscularly, and 16 mg/kg of LiA or dihydro-LiA or equivalent dose of BAL was simultaneously administered by intramuscular injection.