Progesterone binding and inhibition of growth in Trichophyton mentagrophytes

Specific binding of [3H]progesterone to cytosol of Trichophyton mentagrophytes was demonstrated. Scatchard analysis of [3H]progesterone binding showed a single class of binding sites with a dissociation constant of 9.5 X 10(-8) [corrected] +/- 2.4 X 10(-8) M (standard deviation) and a maximal binding capacity of 4,979 +/- 3,489 fmol/mg of cytosol protein. Deoxycorticosterone and dihydrotestosterone competitively inhibited binding by 50% at molar ratios of 10:1 and 20:1, respectively. Other steroid hormones that were tested had minimal activity, indicating binding specificity. Steroid hormone actions in T. mentagrophytes were examined in growth studies. Growth was assessed by determination of cellular ATP content. Progesterone inhibited growth in a dose-responsive manner, with a 50% inhibition concentration of 5.5 X 10(-6) M. Partial recovery from inhibition occurred after 24 to 48 h; inhibition could be enhanced by dividing the amount of added progesterone every 24 h. In the same rank order as was their relationship to each other and progesterone in binding studies, deoxycorticosterone and dihydrotestosterone were less effective inhibitors; other steroid hormones that were tested showed no consistent effect. We hypothesize that the binder described, acting as a hormone receptor, is the molecular site of action for the functional effect of the hormone. The functional effect may be related to the observed resistance of females to dermatophytosis.