Abstract
Progesterone is the naturally-occuring progestational steroid which is inactive when administered orally. It has long been believed that orally administered progesterone is inactivated in the liver before it enters the systemic circulation. While such an inactivation actually occurs in the liver, no evidence has been reported on the stability of progesterone in the small intestine. In this experiment, metabolism of progesterone in the small intestine was studied in rabbits.
A dose of 10 or 20 μc of 3H-progesterone (3.1 μc/μg) was injected into the small intestine of rabbits and the regional mesenterial venous blood was collected for 30-120 min. Extraction of steroids from the serum was carried out as indicated in Fig. 1. While, unexpectedly, preliminary experiments indicated the presence of a considerable amount of radioactivities in the conjugated fraction, more critical examination was carried out as in Fig. 2. Following the extraction of free steroids, conjugated steroids were extracted with butanol and the butanol was evaporated under reduced pressure. Partition of the residue between chloroform and water revealed the presence of almost all the radioactivities in the watre. Recovered radioactivities after hot acid hydrolysis were comparable to that of the butanol extract. These data confirm the presence of conjugated progesterone metabolites in the mesenterial blood.
Both the free and the conjugated steroids obtained as shown in Fig. 1 were then chromatographed on alumina respectively. Fig. 3 shows elution patterns of radioactive steroids in these two steroid fractions. A large proportion of radioactivities was eluted in 0.3% methanol in benzene (MB) fraction and very little was found in the initial 0.1% MB fraction, where, when present, progesterone should be eluted. After chromic acid oxidation of the radioactive steroid eluted in 0.3% MB, the Rf value of the steroid on paper-chromatograph was found to be identical with that of progesterone as seen in Fig. 5. The original steroid is most likely 20-hydroxy-4-pregnene-3-one.
To avoid the influence of bacteria in the intestinal lumen, the metabolism of progesterone was also studied by the “Everted Sac Mrthod” as shown in Fig. 6. After 1-hour incubation, the fluid in the sac was extracted with chloroform. Elution pattern of the steroid in the extract from alumina was almost identical with that of the mesenterial venous blood extract.
All these data indicate that when a small amount of progesterone is administered orally, it must be largely metabolized in the small intestine (probably in the mucosa).
Experiments with a practical amount of progesterone will be reported in the near future.
