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 heart. The following enzyme activities were determined: adenosine kinase (AK), adenosine deaminase (ADA), S-adenosylhomocysteine hydrolase (SAHH), purine nucleoside phosphorylase (PNP), AMP deaminase (AMPD), membrane 5'nucleotidase (M5'N), AMP specific (AC5'N) and IMP specific (IC5'N) cytosolic 5'nucleotidases in (I) rat heart (n = 5), (2) rat cardiomyocytes obtained by collagenase digestion (n = 5), (3) human heart (n = 6) obtained from explants or papillary muscles collected during heart transplantation or mitral Valve replacement, and (4) human umbilical cord endothelial cells in primary culture (n = 4). In the human heart, 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 +/- 0.08), 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 those determined in the rat heart with the exception of AC5'N and IC5'N which were equal. The most prominent difference observed was for AMPD and M5'N which were nine and five-fold more active in the rat heart. Rat cardiomyocyte enzyme activities were comparable to those measured in whole rat heart with the exception of ADA (six-fold lower) and PNP (16-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 myocardial enzymes of purine metabolism are generally less active in the human heart than in rat heart, whereas the crucial pathways of intracellular adenosine production and reincorporation are comparable. The umbilical vein endothelium shows substantial catalytic capacity for adenosine formation both via intracellular and extracellular dephosphorylation 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 predominantly in the endothelium.