NAD+-dependent ADP-ribosyltransferase in renal brush-border membranes

Oxidized nicotinamide adenine dinucleotide (NAD+) in cytosol may interact with renal brush-border membranes (BBM) and inhibit BBM phosphate transport. The possible mechanism of interaction was investigated in the present study. Incubation of BBM with [adenine-3H]NAD+ led to acid-stable binding of 3H to the BBM, in contrast there was no binding of 14C when [carbonyl-14C]NAD+ was used. The data are consistent with an ADP-ribosylation mechanism involving transfer of ADP-ribose from NAD+ to BBM. This was confirmed by using [adenylate-32P]NAD+ and by the release of bound 32P in the form of 5'-[32P]AMP when the BBM were treated with snake venom phosphodiesterase. After gradient centrifugation of BBM the ADP-ribosyltransferase was recovered at the same density as known BBM enzymes, indicating that ADP-ribosyltransferase is an intrinsic BBM component and not a contaminant. These findings indicate that cytosolic NAD+ may be used for ADP-ribosylation of BBM proteins and that this may be a mechanism for regulating the BBM phosphate transport system.