Substrate inhibition and affinities of the glyoxysomal beta-oxidation of sunflower cotyledons

During the glyoxysomal beta-oxidation of long-chain acyl-CoAs, short-chain intermediates accumulate transiently (Kleiter and Gerhardt 1998, Planta 206 : 125-130). The studies reported here address the underlying factors. The s tudies concentrated upon the aspects of (i) chain length specificity and (i i) metabolic regulation of the glyoxysomal beta-oxidation of sunflower (Hel ianthus annuus L.) cotyledons. (i) Concentration-rate curves of the beta-ox idation of acyl-CoAs of various chain lengths showed that the beta-oxidatio n activity towards long-chain acyl-CoAs was higher than that towards short- chain acyl-CoAs at substrate concentrations <20 mu M. At substrate concentr ations >20 mu M, long-chain acyl-CoAs were beta-oxidized more slowly than s hort-chain acyl-CoAs because the beta-oxidation of long-chain acylCoAs is s ubject to substrate inhibition which had already started at 5-10 mu M subst rate concentration and results from an inhibition of the multifunctional pr otein (MFP) of the beta-oxidation reaction sequence. However, low concentra tions of free long-chain acyl-CoAs are rather likely to exist within the gl yoxysomes due to the acyl-CoA-binding capacity of proteins. Consequently, t he beta-oxidation rate towards a parent long-chain acyl-CoA will prevail ov er that towards the short-chain intermediates. (ii) Low concentrations (15 mu M) of a long-chain acyl-CoA exerted an inhibitory effect on the beta-oxi dation rate of butyryl-CoA. Reversibility of the inhibition was observed as well as metabolization of the inhibiting long-chain acyl-CoA. Regarding th e activities of the individual beta-oxidation enzymes towards their C-4 sub strates in the presence of a long-chain acyl-CoA, the MFP activity exhibite d strong inhibition. This inhibition appears not to be due to the detergent -like physical properties of long-chain acylCoAs. The results of the studie s, which are consistent with the observation that short-chain intermediates accumulate transiently during complete degradation of a long-chain acyl-Co A, suggest that the substrate concentration-dependent chain-length specific ity of the beta-oxidation and a metabolic regulation at the level of MFP ar e factors determining this transient accumulation.