Metabolism of Hydroxypyruvate in a Mutant of Barley Lacking NADH-Dependent Hydroxypyruvate Reductase, an Important Photorespiratory Enzyme Activity

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Abstract
A mutant of barley (Hordeum vulgare L.) cv. Klaxon LaPr 88/29, deficient in NADH-dependent hydroxypyruvate reductase (HPR) activity has been isolated. The activities of both NADH (5%) and NADPH-dependent (19%) HPR were severely reduced in this mutant compared to the wild type. Although lacking an enzyme in the main carbon pathway of photorespiration, this mutant was capable of CO2 fixation rates equivalent to 75% of that of the wild type, in normal atmospheres and 50% O2. There also appeared to be little disruption to the photorespiratory metabolism as ammonia release, CO2 efflux and 14CO2 release from L-[U-14C]serine feeding were similar in both mutant and wild-type leaves. When leaves of LaPr 88/29 were fed either [14C]serine or 14CO2, the accumulation of radioactivity was in serine and not in hydroxypyruvate, although the mutant was still able to metabolize over 25% of the supplied [14C]serine into sucrose. After 3 hours in air the soluble amino acid pool was almost totally dominated by serine and glycine. LaPr 88/29 has also been used to show that NADH-glyoxylate reductase and NADH-HPR are probably not catalyzed by the same enzyme in barley and that over 80% of the NADPH-dependent HPR activity is due to the NADH-dependent enzyme. We also suggest that the alternative NADPH activity can metabolise a proportion, but not all, of the hydroxypyruvate produced during photorespiration and may thus form a useful backup to the NADH-dependent enzyme under conditions of maximal photorespiration.