Adenine nucleotides and adenosine metabolism in pig kidney proximal tubule membranes

Exogenous adenosine triphosphate (ATP) added to brush‐border membrane vesicles was rapidly degraded mainly to inosine according to the high ecto‐nucleotidase activities in these vesicles. In the absence of phosphate, inosine was slowly transformed into hypoxanthine, and xanthine oxidase and dehydrogenase activities were not detected. The presence of ecto‐adenosine deaminase and ecto‐adenosine monophosphate (AMP) nucleotidase was shown. The ecto‐adenosine deaminase was inhibited by deoxycoformycin and was also detected in rat renal brush‐border membrane vesicles. Using orthovanadate, levamisole, and α, β‐methylene adenosine diphosphate as possible inhibitors, alkaline phosphatase was shown to be the main agent responsible for ecto‐AMP nucleotidase activity. In pig renal basolateral membrane vesicles and in whole cell extracts from pig renal cortex, ecto‐AMP nucleotidase was the limiting factor in ATP degradation. Comparing the ATP catabolism in the whole cell cortical extract with the catabolism in the same sample precleared of membranes, it was shown that ectonucleotidase activity is mainly bound to the membranous components. It is also shown that the whole cell extract of pig renal cortex has hypoxanthine phosphoribosyl transferase activity, and it seems probable that the rapid and specific formation of luminal inosine and its transport into the cell in competition with adenosine may start the purine salvage pathway through the synthesis of IMP from hypoxanthine. © Wiley‐Liss, Inc.