The distinction between the exposed regions and the buried regions of apoproteins in high density lipoproteins by their reactivities with pronase

Pronase digestion was used to study the surface disposition of apoproteins on high density lipoproteins. After digestion the average density of high density lipoproteins decreased from 1.14 to 1.10 g/ml. The immunoreactivities of apoproteins A-II, C-II, and C-III were completely destroyed, but 80% of the reactivity of ApoA-I was retained. Only 5-10% of ApoA-I reacts with anti ApoAI antisera in intact high density lipoproteins. The similar accessibility of ApoA-I to pronase and to antibodies suggests that pronase hydrolyzes only the exposed regions of protein moieties. Pronase may be an ideal probe for distinguishing the exposed regions of apoproteins in lipoproteins from those that are buried.