PhD

dissertation1. Previous studies indicate that FSH alone does not stimulate estrogen biosynthesis - the end-product of steroidogenesis - in the ovary. The general objective of this study was to determine the effects, if any of FSH devoid of measurable LH biological activity, compared with the effects of FSH combined with LH, on the intermediate steps of steroid biosynthesis in the ovary. 2. The general approach used has been compare in vitro steroidogenic capacity in the following groups: immature mouse ovaries (the control group), ovaries stimulated in vivo with FSH (of both pituitary and urinary origin) - using antisera to neutralize endogenous and exogenous sources of LH - and, finally, ovaries stimulated with the same dose of FSH but with a supplement of LH. Following the in vivo gonadotrophic treatments, homogenates were prepared from ovaries and the capacity to metabolize radioactive steroid substrates investigated by incubating in the presence of added cofactors, and the products analyzed by extraction, chromatography and Recrystallization to constant specific activity. Endogenous steroid levels in the ovarian homogenates were also determined. 3. Testosterone was found to be partially degraded by base catalysis: a finding of methodological importance since separation of steroids with alkali is commonly used in steroid analyses. 4. The steroidogenic profile in the immature mouse ovary revealed the presence of a broad spectrum of enzymes, with very active hydroxysteroid dehydrogenases, moderately active delta4-reductases and low activities of steroidogenic enzymes belonging to the class of mixed function oxidases: 17alph-hydroxylase, 17,20-lyse, cholesterol side-chain cleavage enzyme and aromatizing systems (the latter was immeasurable). 5. The delta4- rather than the delta5 was shown to be the predominant route of androgen formation from pregnenolone, and delta4-androstenedione was preferred to testosterone as androgen precursor of aromatization to estrogens. 6. FSH acted primarily as a stimulator of general growth: it induced and increase in protein synthesis which paralleled the ovarian weight increase, but beyond the general increase in protein synthesis, however, there were not much differential effect on steroidogenic enzymes. 7. The enzyme activities in which FSH alone elicited a modest differential stimulation included the hydroxysteroid dehydrogenases tested. Since, they also included all the enzyme activities markedly enhanced by FSH + LH, this in compatible with previous reports that some LH activity might be an intrinsic property of the FSH molecule. 8. In contrast to FSH alone, FSH + LH elicited marked differential stimulation of specific enzyme activities: 20alph-hydroxysteroid dehydrogenase, aromatizing and cholesterol side-chain cleavage systems. These are therefore critical enzymes of steroid biosynthesis which are under gonadotrophic control. 9. Reflecting in vivo conditions more closely than in vitro studies, endogenous steroid levels revealed that, of the steroids estimated, pregnenolone was the only one slightly stimulated by FSH. LD addition to FSH, on the other hand, increased the levels of all steroids measured