The effect of pyrimidine analogues and tryptophan on enzyme synthesis and degradation in rat liver

Abstract

Summary 5-Fluoroorotic acid, when administered from the zero time point, results in a marked inhibition of the dietary induction of four amino acid-degrading enzymes in rat liver (serine dehydratase, ornithine transaminase, histidase, and tryptophan oxygenase). When tyrosine aminotransferase is induced by the same technique, however, the administration of 5-fluoroorotic acid results in a marked increase in the induced levels of the enzyme. 5-fluoroorotic acid and another pyrimidine analogue, 5-azacytidine, were also shown to stimulate hormonal induction of tyrosine aminotransferase. 5-Fluoroorotic acid was found to be the most potent of several orotic acid analogues studied in the inhibition of the dietary induction of serine dehydratase, and 5-azacytidine was much more effective than 5-fluoroorotic acid on a mg basis. Both analogues also inhibit the hormonal induction of serine dehydratase, and appear to be acting in a manner similar to actinomycin D in that inhibition of enzyme induction occurs only when the administration of the inhibitors is not delayed significantly after the inducers are given. 5-Fluoroortic acid does not inhibit the further induction of tyrosine aminotransferase by cortisone after initial induction by casein hydrolysate in intact animals, but rather prevents the decay of the induced level of the enzyme seen in animals given cortisone alone. In adrenalectomized animals, 5-fluoroorotic acid delays the decay of tyrosine aminotransferase levels induced by casein hydrolysate alone, and actually stimulates the “super-induction” by cortisone. Using immunochemical techniques, it was shown that the enhancement of the hormonal induction of tyrosine aminotransferase by 5-fluoroorotic acid and 5-azacytidine was not related to an increased rate of synthesis, but rather to stabilization of the enzyme. Tryptophan, another efficient inducer of tyrosine aminotransferase, also appears to exert its effect by stabilizing enzyme turnover. The incorporation of 5-fluoroorotic-2-C 14 acid into hepatic RNA and ribonucleoprotein was studied in an effort to gain some insight as to the mechanisms by which this compound influences enzyme induction. 5-fluoroorotate-2-C 14 -labeled cytoplasmic RNA, recently shown to be nonribosomal by chromatographic and sedimentation criteria ( 17 ), was further characterized by studying the polysome-associated ribonucleoprotein in which it is found. The radioactive analogue appears to label selectively a population of ribonucleoprotein which is heterodisperse, nonribosomal, with respect to both sedimentation coefficient and density. The incorporation of 5-fluoroorotic acid into this population of ribonucleoprotein is relatively insensitive to actinomycin D. Possible mechanisms linking base analogues and tryptophan to enzyme turnover are discussed.