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 phos phate, inosine was slowly transformed into hypoxanthine, and xanthine oxidase and dehydrogenase activities were not detected. The presence o f ecto-adenosine deaminase and ecto-adenosine monophosphate (AMP) nucl eotidase was shown. The ecto-adenosine deaminase was inhibited by deox ycoformycin and was also detected in rat renal brush-border membrane v esicles. Using orthovanadate, levamisole, and alpha, beta-methylene ad enosine diphosphate as possible inhibitors, alkaline phosphatase was s hown to be the main agent responsible for ecto-AMP nucleotidase activi ty. In pig renal basolateral membrane vesicles and in whole cell extra cts from pig renal cortex, ecto-AMP nucleotidase was the limiting fact or 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 ecto-nucleotidase activity is mainly boun d to the membranous components. It is also shown that the whole cell e xtract 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 wit h adenosine may start the purine salvage pathway through the synthesis of IMP from hypoxanthine. (C) 1993 Wiley-Liss, Inc.