ADP-Ribosylation in Mitochondria: Enzymatic and Non Enzymatic Reactions

Abstract

Mitochondria are known to be able to synthesize NAD⁺, the coenzyme involved in various oxidation systems (1, 2). Moreover it has been demonstrated that NAD⁺ is a substrate for enzymes such as ADP-ribosyl transferases and NAD glycohydrolases. ADP-ribosylation in mitochondria was first described by Kun and co-workers (3). Enzymatic transfer of ADP- ribose from NAD⁺ to an acceptor protein of 50 kDa has been observed in mitochondrial extracts. Richter et al. have reported a covalent modification by mono (ADP-ribosyl)ation of a 31 kDa protein present in submitochondrial particles (SMP) which seems to be involved in the regulation of Ca²⁺ release from mitochondria (4). According to Hilz et al. (5) non enzymatic ADP-ribosylation may occur in beef heart and liver mitochondria after enzymatic hydrolysis of NAD⁺ by NAD glycohydrolase and subsequent transfer of ADP-ribose to specific acceptors (5). We recently demonstrated (6) the existence of ADP-ribosyl transferase activity in rat liver mitochondria and mitoplasts. While NAD glycohydrolase activity was found to be inhibited by nicotinamide or 3-aminobenzamide, ADP-ribosyl transferase activity was found to be unaffected. ADP-ribose bound to mitochondrial proteins appears to be oligomeric. In this paper we demonstrate that removal of NAD glycohydrolase from solubilized mitochondrial proteins by hydroxyapatite chromatography does not affect the transfer of ADP-ribose from NAD⁺.