| Abstract: | Previous studies of serine dehydratase (EC 4.2.1.13) and ornithine aminotransferase (EC 2.6.1.13) adaptation in rat liver showed that in rats on a high protein diet, glucocorticoid administration increased serine dehydratase activity while simultaneously reducing the activity of ornithine aminotransferase. The present study examines the role of enzyme synthesis in the expression of these and other dissimilar adaptive characteristics of the two enzymes. Both enzymes were purified to crystallinity and used to prepare specific antibodies. Changes in the rate of synthesis of each enzyme during adaptation were then measured immunochemically. In rats fed ad libitum, the synthetic rates for both enzymes exhibited circadian rhythm, although enzyme levels remained relatively constant. The circadian cycle for ornithine aminotransferase synthesis was in phase with the cycles for body weight and relative liver weight (maxima at 9 a.m., minima at 9 p.m.) but was approximately 12 hours out of phase with the cycle for serine dehydratase synthesis. 9alpha-Fluoro-11beta, 21-dihydroxy-16alpha, 17alpha-isopted at 9 a.m., increased serine dehydratase synthesis and simultaneously decreased the synthesis of ornithine aminotransferase. When triamcinolone was injected at 9 p.m., however, serine dehydratase synthesis was not stimulated, although the reduction of ornithine aminotransferase synthesis was still produced. These results suggest that: (a) circadian cycling of synthesis may be a general phenomenon in enzyme regulation even though for enzymes with relatively long half-lives, such cycling may not be reflected as fluctuations in enzyme levels; (b) such circadian rhythmicity may also involve cyclic changes in the responsiveness of the enzyme-forming system to regulatory stimuli; (c) whereas the adaptive behavior of serine dehydratase typifies that of amino acid-catabolizing enzymes in general, the responses of ornithine aminotransferase denote a functional association of this enzyme with anabolic processes. On this basis, the possibility that ornithine aminotransferase plays a pivotal role in the regulation of urea cycle activity and nitrogen balance is discussed. |
