Specific labeling of lens aldehyde dehydrogenase class 1 from (3)H-cholesterol or its derivatives

We investigated the possibility that sterols could covalently modify ocular lens cell proteins. Incubation of cultured bovine lens epithelial cells (BLEC) with (3)H-cholesterol led to the labeling of a cytosolic protein of about 52 kD. Two-dimensional electrophoresis of the BLEC soluble proteins and fluorography revealed one labeled protein of 52 kD, pI = 6.6, plus a weakly labeled, slightly more acidic protein of the same size. MALDI-MS analysis of both proteins recovered from duplicate gels indicated both to be aldehyde dehydrogenase class 1 (ALDH-1). The identity was confirmed by immunoprecipitation with antiserum to ALDH-1. Alkaline hydrolysis of (3)H-labeled ALDH-1 released most of the radiolabel as compounds much more polar than cholesterol. We speculate that lens ALDH-1 can participate in the oxidation of cholesterol or its derivatives to unidentified sterol carboxylic acids and that the labeled protein reflects capture of ALDH-1 with sterol intermediates covalently bound to the enzyme in ester linkage. Lens ALDH-1 might, therefore, participate in the detoxication of polar sterols.