Abstract
[4-14C]-Labelled pregnenolone, progesterone, 17α-hydroxyprogesterone, androstene-dione and testosterone were incubated with 630-day-old White Leghorn cock testicular homogenate in the presence of 0, 10-6, 10-5 and 10-4 M melatonin. Activities of Δ5-3β-hydroxysteroid dehydrogenase, Δ5-4 isomerase and 17α-hydroxy-C17-20 lyase were strongly inhibited by the addition of 10-6-10-4 M melatonin in the medium. Melatonin inhibits the biosynthesis of testosterone in the testis of the chicken. The activities of 17α-hydroxylase and 17β-hydroxysteroid dehydrogenase were not inhibited by melatonin in the present study. The activity of almost all the enzymes concerning testosterone synthesis in the chicken testis was significantly higher in the light (11:00) period than the dark (23:00) period. The lower activities of the enzymes in dark time may be due to a higher secretion of melatonin which is supressive to testerone synthesis.
