Purine nucleotide catabolism regulation

Matsuda et al. (27) showed that the adenylosuccinate synthetase basic isozyme has a lower Km for aspartate, is more sensitive to inhibition by fructose 1,6-bisphosphate, and less sensitive to inhibition by nucleotides than the acidic isozyme. These properties could indicate that the basic isozyme is regulated coordinately with glycolysis (or gluconeogenesis) as proposed for the operation of the purine nucleotide cycle in skeletal muscle. The enzyme could also be affected by the availability of aspartate, as was found in Ehrlich ascites cells. The increase in basic isozyme activity, under conditions used in this study where the animal must rely on protein for most of its energy, is consistent with the idea that it is involved in the purine nucleotide cycle. This probably is not as an alternative to glutamate dehydrogenase in urea synthesis but is simply in amino acid catabolism. The small... 

The importance of blood and, in particular, of erythrocytes as a vehicle for transport of purines is well known >. Considerable quantities of purines enter and leave the nucleotide pools of red cells which take up adenine, guanine, hypoxanthine, and xanthine and convert them into nucleotides. No matter what purine is taken up by erythrocytes, hypoxanthine appears to be the main purine released in vivo. Human erythrocytes cannot synthesize purines de novo and are unable to convert hypoxanthine or guanine into adenine. Hypoxanthine release is mediated by prior conversion of the various purine nucleotides to IMP. In the present paper, some of the mechanisms which regulate the catabolic paths of this nucleotide are studied. 

Information on cellular metabolic organization of caffeine biosynthesis and catabolism links to purine nucleotide metabolism, intercellular translocation, and accumulation mechanisms at specific cellular sites, such as chloroplasts and vacuoles, have yet to be fully revealed. Cell-, tissue-, and organ-specific synthesis and possibly catabolism of purine alkaloids may be regulated by unique and unknown developmental- and environmental-specific control mechanisms. A great deal of fascinating purine alkaloid biology in plants still remained to be discovered. 

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