The dephosphorylation of AMP and IMP by a soluble low Km 5'nucleotidase from human seminal plasma: some regulatory aspects.

In this study, a soluble low Km 5'nucleotidase, dephosphorylating IMP with a Vmax/Km ratio 10-times higher than that of AMP, has been purified from human seminal plasma. The effect of inorganic phosphate (Pi) and adenylate energy charge variations on the activity of this enzyme has also been investigated. In the physiological range, with IMP as substrate, the activity of the enzyme does not change whereas the hydrolysis of AMP increases with decreasing energy charge values. In the presence of both the substrates, phosphate exerts an inhibitory effect on the enzyme activity with a similar concentration dependence pattern. The results show that AMP-hydrolysing activity responds to variations of energy charge by increasing the AMP degradation thus protecting the value of energy charge at the expense of a decrease in the total adenylate pool. In contrast, the dephosphorylation of IMP is not regulated by changes in energy charge. This data suggests that the degradation of IMP and AMP, although carried out by the same enzyme, is controlled by different regulatory mechanisms.