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 |
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Authors: | S Blom L Förlin TB Andersson |
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Institution: | (1) Department of Zoophysiology, University of Göteborg, Box 463, SE 405 30 Göteborg, Sweden;(2) AstraZeneca R&D, S-431 83 Mölndal, Sweden |
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Abstract: | The formation of 11-deoxycortisol from 17-hydroxyprogesterone was studied in rainbow trout (Oncorhynchus mykiss) head kidney and liver microsomes. The K
m for head kidney microsomal 17-hydroxyprogesterone 21-hydroxylase measured in pooled microsomes from juvenile rainbow trout was 3.75 M, and the V
max was 41.4 pmol/(mg protein × min). The K
m for liver microsomal 17-hydroxyprogesterone 21-hydroxylase was 31.0 M, and the V
max was 51.0 pmol/(mg protein × min). The intrinsic clearance values suggest that liver, in addition to head kidney, could significantly contribute to the formation of 11-deoxycortisol in rainbow trout. Progesterone inhibited both the head kidney and the liver microsomal 17-hydroxyprogesterone 21-hydroxylase activity competitively. The K
i for progesterone in head kidney was 1.8 M and in liver 144 M. The different K
m and K
i values for microsomal 21-hydroxylation suggest that 21-hydroxylation of 17-hydroxyprogesterone in head kidney and liver is catalysed by different enzyme systems. Treatment of rainbow trout by adrenocorticotrophic hormone (ACTH) for 1.5 or 3 h (single injections) or three weeks (injections every other day) did not affect head kidney or liver17-hydroxyprogesterone 21-hydroxylase activity. ACTH may thus not regulate this enzyme activity. The increase in plasma cortisol levels 1.5 and 3 h after ACTH injection but not following repeated injections suggests that the hormone regulates the initial secretion of cortisol, but does not have a long-term regulatory effect upon cortisol production in rainbow trout. |
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Keywords: | adrenocorticotrophic hormone cortisol corticosteroid production enzyme kinetics 21-hydroxylation |
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