A comparison of nutritional and genetic blocks in the synthesis of purines by yeasts, molds and bacteria

REGULATION AND COORDINATION OF PURINE AND PYRIMIDINE BIOSYNTHESES IN YEAST. I. REGULATION OF PURINE BIOSYNTHESIS AND ITS RELATION TO TRANSIENT CHANGES IN INTRACELLULAR NUCLEOTIDE LEVELS

The control of purine biosynthesis in a yeast mutant deficient for uracil, adenine, and histidine has been studied in vivo. The adenine mutation causes accumulation of aminoimidazole ribotide in the cells. The control curve relating steady-state purine nucleotide level in the cell to rate of synthesis in the de novo purine synthetic pathway has been determined. Control in the cell depends on a feedback mechanism involving end-product inhibition. The transient responses of the purine nucleotide pool to changes in adenine input have been studied. Under certain conditions the pool overshoots when shifting from one steady-state to another. Transient changes in nucleotide levels are followed by inverse changes in the rate of attempted de novo purine synthesis. A study of the transient responses of specific intracellular nucleotides suggests that inosinic acid controls the rate of attempted purine synthesis. The transient response of nucleic acid synthesis rate to changes in nucleotide levels was studied and the implications for regulation of nucleic acid synthesis discussed.

Aminoimidazole ribonucleotide carboxylase. Partial purification and properties

The enzyme aminoimidazole ribonucleotide carboxylase has been partially purified from chicken livers. A somewhat higher degree of purification was achieved with pigeon liver acetone powder as the starting material. This enzyme carboxylates the ribonucleotide but cannot use the free base, aminoimidazole, as its substrate. An investigation of the requirements for optimal activity reveals that magnesium ions are essential. Inhibition by a variety of reagents which react with sulfhydryl groups indicates their importance in the fixation of carbon dioxide to aminoimidazole ribonucleotide. An exhaustive search revealed that none of the B-vitamin cofactors play a role in this reaction. The lack of an energy requirement, e.g. ATP, for activity and the nucleophilic nature of the amino-substituted ring suggest that the carboxylation of aminoimidazole ribonucleotide proceeds via a direct enzyme-mediated reaction involving Mg++ions and active sulfhydryl groups.

NITRO- AND AMINO-IMIDAZOLESULPHONAMIDES

Several 4(5)-nitro- and 4(5)-amino-5(4)-sulphamylimidazoles have been synthesized as potential antagonists to 4(5)amino-5(4)-carbamylimidazole.