Enzyme crypticity as an indicator of membrane orientation in envelope vesicles from halobacteria

A membrane fraction prepared by freeze-thawing envelope vesicles from Halobacterium halobium exhibits menadione-dependent NADH dehydrogenase activity (EC 1.6.9.9.3). The activity is “unmasked” by Triton X-100. Activity in freeze-thawed membrane preparations is more firmly membrane-bound than in sonicated vesicle preparations. Affinity for NADH is higher than in untreated vesicles and pyridine adenine dinucleotide specificity is the same. The data indicate that freeze-thawing causes reintegration of an adsorbed, extrinsic form of the dehydrogenase to yield an intrinsic, more native-like form of the enzyme. It is concluded that the extrinsic NADH dehydrogenase activity in vesicle preparations is not directly related to vesicle orientation.