NUCLEOTIDE AND ADENOSINE METABOLISM IN DIFFERENT CELL-TYPES OF HUMAN AND RAT-HEART

Evaluation of enzyme activities involved in nucleotide metabolism and adenosine production within different cell types can provide important information on their contribution to the overall metabolism of the he art. The following enzyme activities were determined: adenosine kinase (AK), adenosine deaminase (ADA), S-adenosylhomocysteine hydrolase (SA HH), purine nucleoside phosphorylase (PNP), AMP deaminase (AMPD), memb rane 5'nucleotidase (M5'N), AMP specific (AC5'N) and IMP specific (IC5 'N) cytosolic 5'nucleotidases in (I) rat heart (n = 5), (2) rat cardio myocytes obtained by collagenase digestion (n = 5), (3) human heart (n = 6) obtained from explants or papillary muscles collected during hea rt transplantation or mitral Valve replacement, and (4) human umbilica l cord endothelial cells in primary culture (n = 4). In the human hear t, activities (mu mol/min/g wet weight) were as follows: AK (0.14 +/- 0.01), ADA (0.46 +/- 0.03), SAHH (0.001 +/- 0.0003), PNP (0.43 +/- 0.0 8), AMPD (0.41 +/- 0.05), M5'N (1.75 +/- 0.12), IC5'N (0.21 +/- 0.03) and AC5'N (0.11 +/- 0.02), These enzyme activities were lower than tho se determined in the rat heart with the exception of AC5'N and IC5'N w hich were equal. The most prominent difference observed was for AMPD a nd M5'N which were nine and five-fold more active in the rat heart. Ra t cardiomyocyte enzyme activities were comparable to those measured in whole rat heart with the exception of ADA (six-fold lower) and PNP (1 6-fold lower). Endothelial cell activities were notably different from those in the human heart particularly in the case of SAHH (nine-fold higher) and PNP (16-fold higher). These data highlight the fact that m yocardial enzymes of purine metabolism are generally less active in th e human heart than in rat heart, whereas the crucial pathways of intra cellular adenosine production and reincorporation are comparable. The umbilical vein endothelium shows substantial catalytic capacity for ad enosine formation both via intracellular and extracellular dephosphory lation and SAHH pathways. Comparison of activities in endothelial cell preparations with those in the whole heart indicate that the enzymes of adenosine breakdown pathway, ADA and PNP, appear to be located pred ominantly in the endothelium.