Regulation of Purine Ribonucleotide Synthesis by End Product Inhibition

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Abstract
1. Succino-AMP synthetase (EC 6.3.4.4) was partially purified from the adenine starved cells of a Bacillus subtilis mutant which was derived from K strain and lacking succino-AMP; YASE (ec 4.3.2.2). The enzyme was stablizied by the presence of 1 mM dithiothreitol. Optimum pH for activity was about 7.5. Apparent Michaelis constants for IMP, GTP and L-aspartic acid were 34μM, 73 μM and 680μM, respectively. 2. The enzyme was inhibited strongly by the end products, AMP and ADP. Complete inhibition was obtained by 0.3mM AMP. This inhibition by AMP was competitive to GTP, noncompetitive to L-aspartic acid and of a mixed type to IMP. Both the Lineweaver-Burk plots in the presence of AMP and rate-AMP concentration curve were normal. Succino-AMP, GDP and inorganic orthophosphate, which are the direct product of the reaction, were also inhibitory at higher concentrations.

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