Abstract
Members of the AKR7A family of aldo-keto reductases have previously been proposed to act as succinic semialdehyde reductases in brain and in liver. Human AKR7A2, rat AKR7A4 and mouse AKR7A5 all have high affinity for succinic semialdehyde (SSA) (K-m=10-20 mu M), and may play an important role in the biosynthesis of gamma-hydroxybutyrate (GHB), an endogenous metabolite derived from SSA. The catabolism of GHB has attracted much attention recently particularly because of its use as a "date rape" drug or drug of abuse. It is known to be rapidly metabolised in humans, but the genes encoding the enzymes responsible have not been identified. We have investigated the ability of mouse AKR7A5 to act as a GHB dehydrogenase in order to evaluate its role in the conversion of GHB to SSA. AKR7A5 appears to be a very poor GHB dehydrogenase in vitro, with an apparent Km of 3.2 mM and k(cat) of 1.85 min(-1). However, AKR7A5 could act as a GHB-DH in vivo if the reaction is coupled to the oxidation of SSA to succinate by succinic semialdehyde dehydrogenase (SSADH) or GABA transaminase (GABA-T).
GHB dehydrogenases were originally considered to be predominantly cytosolic. However, SSADH and GABA-T have previously been shown to be mitochondrial. For effective coupling of GHB and SSA oxidation in vivo, the enzymes would need to be co-localized. We have determined that human AKR7A2 is located in the mitochondria in the neuroblastoma cell line SH-SY5Y, and we propose that this localization is mediated through the presence of additional amino terminal targeting signals.