Induction of enzyme synthesis

Induction of enzyme synthesis Glucagon increases the tran scription of the PEP carboxykinase gene, thereby increasing the availability of this enzyme s activity as levels of its substrate rise during fasting. [Note Insulin causes decreased transcription of the mRNA for this enzyme.]... 

Figure 11-1 Some control elements for metabolic reactions. Throughout the book modulation of the activity of an enzyme by allosteric effectors or of transcription and translation of genes is indicated by dotted lines from the appropriate metabolite. The lines terminate in a minus sign for inhibition or repression and in a plus sign for activation or derepression. Circles indicate direct effects on enzymes, while boxes indicate repression or induction of enzyme synthesis.

Biotin is the coenzyme in a small number of carboxylation reactions in mammalian metabolism and some decarboxylation and transcarboxylation reactions in bacteria. Although the biotin-dependent enzymes are cytosolic and mitochondrial, about 25% of tissue biotin is found in the nucleus, much of it bound as thioesters to histones. Biotin has two noncoenzyme functions induction of enzyme synthesis and regulation of the cell cycle. 

Induction of enzymes, synthesis of enzyme in response to the exposure of organisms to a specific substrate. 

Conney AH, Miller EC, Miller JA. 1956. The metabolism of methylated aminoazo dyes. V. Evidence for induction of enzyme synthesis in the rat by 3-methyl-cholanthrene. Cancer Res. 16 450-59... 

Induction of enzyme synthesis requires activation of transcription factors. One of the factors that is activated is CREB, which stands for cAMP response element binding protein. CREB is phosphorylated and activated by the cAMP-dependent protein kinase, which itself is activated on glucagon or epinephrine stimulation. A second set of transcription factors activated are the C/EPBs (CCAAT/ enhancer binding proteins), although the regulation of their activity is still under investigation. 

Fig. 42.5. Hormonal regulation of hepatic amino acid metabolism in the postabsorptive state. Circled + = glucagon-mediated activation of enzymes or proteins circled t = induction of enzyme synthesis mediated by glucagon and glucocorticoids. Induction of urea cycle enzymes occurs both during fasting and after a high-protein meal. Because many individuals in the United States normally have a high-protein diet, the levels of urea cycle enzymes may not fluctuate to any great extent.

Activation of lipoprotein lipase at the cell surface. As shown in Table 9.2, lipoprotein lipase has a very short half-life, of the order of 1 hour. In response to insulin acting on adipocytes there is induction of enzyme synthesis. The newly synthesized enzyme then migrates to the surface of the blood vessel endothelial walls, where it binds chylomicrons or VLDL (section 5.6.2) and catalyses the hydrolysis of triacylglycerol. The non-esterified fatty acids are mainly taken up by adipose tissue and used for synthesis of triacylglycerol. 

Fig. 99. Stimulation of nuclear RNA synthesis during induction of enzyme synthesis by administration of hydrocortisone (Kenney and Kull, 1963).

Three little-known-facts about chloroplast lipids should be familiar to all plant lipldologists. First is the role of galactollplds in the historical development of molecular biology. When Monod and Jacob (Monod et al., 1952) discovered induction of enzyme synthesis (Pardee et al.,... 

DeLapp and Fisher (7) showed marked elevation of the liver concentrations of hypoxanthine plus xanthine (H + X) and suggested that the mechanism of induction could involve substrate induction of enzyme synthesis. Stirpe and Della Corte (1) have shown that adenine induces an increase in the level of liver XDH. Woodward et al. (8) confirmed this observation and showed that the level of H + X was also elevated. It was shown that when adenine and allopurinol were fed together a 6-fold increase in activity could be obtained, with the tissue levels of H + X elevated to approximately 40 mgs per 100 gs tissue (allopurinol alone gave values of 15-20 mg % and controls were generally 1-4 mg %). It... 

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