We describe the effects of methylenecyclopropylglycine in fasted rats. A 75% decrease in the blood glucose concentration and an increase of lactate and pyruvate were observed 6 h after administration of 100 mg of this amino acid/kg. By contrast with the effects reported for hypoglycin [Williamson & Wilson (1965) Biochem. J. 94, 19c-21c], the plasma concentrations of ketone bodies decreased after administration of methylenecyclopropylglycine and the concentrations of branched-chain amino acids in the plasma were increased 6-fold. The oxidation of decanoylcarnitine or of palmitate was nearly completely inhibited in rat liver mitochondria from methylenecyclopropylglycine-poisoned rats. The activities of acetoacetyl-CoA and of 3-oxoacyl-CoA thiolase were decreased to 25% and less than 10% of the controls. There was a pronounced aciduria, due to the excretion of dicarboxylic acids and of oxidation products of branched-chain amino acids. The accumulation of the toxic metabolite methylenecyclopropylformyl-CoA in the mitochondrial matrix was detected after administration of methylenecyclopropylglycine. Similarly we confirmed experimentally that methylenecyclopropylacetyl-CoA accumulates in mitochondria incubated with methylenecyclopropylpyruvate.

The effects of methylenecyclopropylglycine (MCPG), the lower homologue of hypoglycin A, on starved rats are described. Upon oral ingestion of MCPG (43 mg/kg), a 50% decrease in blood glucose compared with controls was observed after 4 h. The plasma concentrations of lactate and non-esterified fatty acids were substantially increased during this period. The activity of general acyl-CoA dehydrogenase from isolated rat liver mitochondria was not significantly changed. By contrast, the activity of 2-methyl-(branched-chain)-acyl-CoA dehydrogenase decreased by over 80%. The enzyme activity of enoyl-CoA hydratase (crotonase) from pig kidneys decreased by 80% on incubation with the hypothetically toxic metabolite of MCPG, methylenecyclopropylformyl-CoA. These results suggest that the inhibition spectrum of MCPG is quite different from that of hypoglycin A and that similar physiological effects might result from inhibition of different enzymes of beta-oxidation, e.g. hypoglycaemia and lacticacidemia. Accumulation of medium-chain acyl-CoA thioesters is probably at the origin of disturbances in pyruvate metabolism.

Acyl-CoA dehydrogenases are flavoproteins involved in the degradation of fatty acids and of branched chain amino acids. Their reaction mechanism is assumed to involve a concerted n,ß-elimination, starting with abstraction of the a-proton [1]. Incubation of medium and short chain acyl-CoA dehydrogenases (MCADH and SCADH) with 2-octynoyl-CoA leads to covalent modification of the enzyme active site [2,3]. The amino acid involved is Glu 376 , which has been proposed to be the base abstracting the hydrogen as an a-proton [4]. Since Glu 376 is not congerved in all acyl-CoA dehydrogenases (Isovaleryl-CoA and LCADH have Gly at position 376) [5], the above mentioned role of Glu 376 can be questioned. In order to investigate the role of Glu 376 , we studied the reactivity with 2-octynoyl-CoA of two enzymes lacking Glu 376. These are long chain acyl-CoA dehydrogenase (LCADH) which has a Gly376 [5], and the Glu 376-Gln mutant of human MCADH [6]., Results and Discussion: Reaction of 2-octynoyl-CoA with LCADH: The incubation of LCADH with 2-octynoyl-CoA leads to changes of the oxidized flavin spectrum which are similar to those reported by Freund et ale [3] using MCADH. A difference is the complete Flavins and Flavoproteins 1990

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Acyl-CoA dehydrogenases have gained considerable interest in the past few years. This was due to a large extent to the discovery of genetic defects in the B-oxidation cycle and their connection with the "sudden infant death syndrome" (SIDS) [I]. Up to date, several cases of genetic defects involving a-B-dehydrogenation and in particular the enzyme medium chain acyl-CoA dehydrogenase have been reported [2,3].The occurrence of such defects involving the short chain-and long chain acyl-CoA dehydrogenases is more limited.