Inhibition of 3α-hydroxysteroid dehydrogenase (3α-HSD) activity of human lung microsomes by genistein, daidzein, coumestrol and C 18-, C 19- and C 21-hydroxysteroids and ketosteroids

Abstract

Epidemiologic data suggest a relationship between dietary intake of phytochemicals and a lower incidence of some cancers. Modulation of steroid hormone metabolism has been proposed as a basis for this effect. It has been shown that aromatase, 3β-hydroxysteroid dehydrogenase and 17β-hydroxysteroid dehydrogenase (17β-HSD) are inhibited by the isoflavones, genistein and daidzein, and by coumestrol. In general, the extent of inhibition has been expressed in terms of IC 50-values, which do not give information as to the pattern of inhibition, i.e., competitive, non-competitive, or mixed. Less is known of the effects of these compounds on 3α-HSD. The human lung is known to have a high level of 17β-HSD and 3α-HSD activity. During the course of studies to characterize both activities in normal and inflamed lung and lung tumors we noted that 3α-HSD activity with 5α-DHT of microsomes from normal, adult lung was particularly susceptible to inhibition by coumestrol. To clarify the pattern of inhibition, the inhibition constants K i and K ′ i were evaluated from plots of 1 / v versus [ I] and [ S ] / v versus [ I]. Genistein, daidzein and coumestrol gave mixed inhibition patterns versus both 5α-DHT and NADH. In contrast, 5α-androstane-3,17-dione and 5α-pregnane-3,20-dione were competitive with 5α-DHT. NAD inhibited competitively with NADH. Our findings demonstrate that phytochemicals have the potential to inhibit 5α-DHT metabolism and thereby affect the androgen status of the human lung. The observation of a mixed inhibition pattern suggests these compounds bind to more than one form of the enzyme within the catalytic pathway.