Characterisation of head kidney and liver microsomal 17-hydroxyprogesterone 21-hydroxylase activity in rainbow trout (Oncorhynchus mykiss) and the lack of regulatory effects of ACTH

The formation of 11-deoxycortisol from 17-hydroxyprogesterone was studied i n rainbow trout (Oncorhynchus mykiss) head kidney and liver microsomes. The K-m for head kidney microsomal 17-hydroxyprogesterone 21-hydroxylase measu red in pooled microsomes from juvenile rainbow trout was 3.75 muM, and the V-max was 41.4 pmol/(mg protein x min). The K-m for liver microsomal 17-hyd roxyprogesterone 21-hydroxylase was 31.0 muM, and the V-max was 51.0 pmol/( mg protein x min). The intrinsic clearance values suggest that liver, in ad dition to head kidney, could significantly contribute to the formation of 1 1-deoxycortisol in rainbow trout. Progesterone inhibited both the head kidn ey and the liver microsomal 17-hydroxyprogesterone 21-hydroxylase activity competitively. The K-i for progesterone in head kidney was 1.8 muM and in l iver 144 muM. The different K-m and K-i values for microsomal 21-hydroxylat ion suggest that 21-hydroxylation of 17-hydroxyprogesterone in head kidney and liver is catalysed by different enzyme systems. Treatment of rainbow tr out by adrenocorticotrophic hormone (ACTH) for 1.5 or 3 h (single injection s) or three weeks (injections every other day) did not affect head kidney o r liver17-hydroxyprogesterone 21-hydroxylase activity. ACTH may thus not re gulate this enzyme activity. The increase in plasma cortisol levels 1.5 and 3 h after ACTH injection but not following repeated injections suggests th at the hormone regulates the initial secretion of cortisol, but does not ha ve a long-term regulatory effect upon cortisol production in rainbow trout.