Near‐Stoichiometric interaction between the non‐specific lipid‐transfer protein of the yeast Candida tropicalis and peroxisomal acyl‐coenzyme A oxidase prevents the thermal denaturation of the enzyme in vitro

A 14‐kDa peroxisomal‐matrix protein, named PXP‐18, of the yeast Candida tropicalis is a structural and functional homologue of the mammalian nonspecific lipid‐transfer protein (identical to sterol carrier protein‐2). PXP‐18 protected acyl‐coenzyme A oxidase (ACO), the rate limiting enzyme of the peroxisomal β‐oxidation of fatty acids, from thermal inactivation at 48°C or 70°C. This effect was dose‐dependent and not replaceable either by chicken egg white lysozyme, which is similar to PXP‐18 (insofar as it is basic, small, and monomeric), or by bovine serum albumin, a carrier of lipids in the blood. ACO was irreversibly denatured by heat treatment at 70°C for 15 min. However, when ACO and PXP‐18 were similarly heat‐treated, they formed a large complex at a molar ratio of PXP‐18 to ACO subunit that was about one, independent of their initial ratio. This near‐stoichiometric complex had ACO activity after a 500‐fold dilution and was accompanied by ACO that was free of PXP‐18 and indistinguishable from native ACO in size and activity. PXP‐18 also protected urate oxidase, another peroxisomal enzyme, from inactivation at 66°C for 15 min and facilitated the renaturation of ACO denatured by 2 M urea. These results indicated that PXP‐18 is active in modulating the structure of peroxisomal enzymes in vitro . It is possible that PXP‐18 functions as a stress protein or as a part of the system that keeps peroxisomal proteins intact.