Kinetics of adenylate metabolism in human and rat myocardium

Pathways producing and converting adenosine have hardly been investigated in human heart, contrasting work in other species. We compared the kinetics of enzymes associated with purine degradation and salvage in human and rat heart cytoplasm assaying for adenosine deaminase, nucleoside phosphorylase, xanthine oxidoreductase, AMP deaminase, AMP- and IMP-specific 5'-nucleotidases, adenosine kinase and hypoxanthine guanine phosphoribosyltransferase (HGPRT). Xanthine oxidoreductase was not detectable in human heart. The Km-values of the AMP-catabolizing enzymes were 2-5 times higher in human heart; the substrate affinity of the other enzymes was in the same order of magnitude in both species. The maximal activity (Vmax) of adenosine kinase was the same in both species, but HGPRT in man was only 12% of that in the rat. For human heart the Vmax-values of adenosine deaminase, nucleoside phosphorylase, AMP- and IMP-specific 5'-nucleotidases, and AMP deaminase were 25-50% of those for rat heart. We conclude that human heart is less geared to purine catabolism than rat heart as is evident from the lower activities of the catabolic enzymes. Maintenance of the nucleotide pool may thus play a more important role in human heart.