Acetylation of 13-sophorosyloxydocosanoic acid by an acetyltransferase purified from Candida bogoriensis.

Abstract

Acetyl-coenzyme A: 13-sophorosyloxydocosanoic acid (Glc2HDA) acetyltransferase was purified 14-fold in low yield from Candida bogoriensis cells. The enzyme catalyzes acetylation of the 6' and 6" positions of the sophorosyl group, producing the 13-[2'-O-beta-D-glucopyranosyl-beta-D-glucopyranosyloxy]-docosanoic acid 6',6"-diacetate (Ac2Glc2HDA) and monoacetate (AcGlc2HDA) in a product ratio of 5:1. Neither the purification steps nor heat denaturation studies indicated separation of the first and second acetylation steps. The acetyltransferase has a molecular weight of about 500,000 as determined by gel filtration on a Sepharose 4-B column. It shows a pH optimum range from 7 to 9, is strongly inhibited by 1 mM concentrations of the sulfhydryl reagents N-ethylmaleimide, p-hydroxymercuribenzoate, and 5,5'-dithiobis(2-nitrobenzoic acid), but only partly inhibited by 10 mM iodoacetamide. It has an apparent Km of 30 muM for acetyl-CoA, utilizes propionyl-CoA at 45% the rate of acetyl-CoA, and utilizes longer chain acyl-CoA derivatives much less efficiently. The critical micelle concentrations of the C. bogoriensis glycolipids in pH 7.7 phosphate buffer were estimated by pinacyanol chloride binding as follows: Glc2HDA, 50 mum; AcGlc2HDA, 30 muM; Ac2Glc2HDA, 12 muM. The Stokes radius of Ac2Glc2HDA micelles was 22 A as estimated by gel filtration on Bio-Gel P-150. Glc2HDA was a much better acceptor than its methyl ester in the acetyltransferase assay. A plateau in the Glc2HDA saturation curve at 50 muM and a corresponding break in the reciprocal plot at this concentration indicate the enzyme utilizes the monomeric form of this lipid as substrate.