Palmitic acid is associated with halorhodopsin as a free fatty acid. Radiolabeling of halorhodopsin with 3H-palmitic acid and chemical analysis of the reaction products of purified halorhodopsin with thiols and NaBH4.

Halorhodopsin, isolated from Halobacterium salinarium cells incubated with tritiated palmitic acid, co-elutes with labeled palmitate in phenylsepharose CL-4B chromatography. Halorhodopsin-bound 3H-palmitate is not readily displaced by prolonged exposure to a large excess of detergents and by re-chromatography of radiolabeled halorhodopsin on phenylsepharose. On other hand, the association of labeled palmitate with purified halorhodopsin is not resistant to denaturation induced either by isopropanol/hexane or by SDS gel electrophoresis. We have tested the hypothesis that tightly associated palmitate is bound to halorhodopsin through a thioester bond, which is unstable in denaturing conditions. Using GC/MS, we have analysed the reaction products of native halorhodopsin with specific thioester reagents, thiols and NaBH4, which are inactive on free fatty acids. The results of this analytical approach indicate that there is no thioester bond between halorhodopsin and palmitic acid and that palmitic acid is associated with halorhodopsin as a free fatty acid. Copyright 1998 Elsevier Science B.V.