The effect of pH on the hydrolysis of chylomicron and chylomicron remnant cholesterol ester with rat liver homogenate was examined. The hydrolysis had three pH optima, at pH 4.5, at pH 6.0-6.5 and at pH 8.5. At the two upper pH optima extensive cholesterol ester hydrolysis occurred without simultaneous degradation of the triacylglycerol portion. 2. Similarly, microsomes (at pH 6.5-8.0) and 100 000 X g supernatant (at pH 7.5-8.5) efficiently hydrolyzed the cholesterol ester but not the triacylglycerol of chylomicron remnants. 3. With the same substrate no enrichment of neutral cholesterol esterase activity was seen in isolated plasma membranes. 4. At pH 4.5 lysosomes efficiently hydrolyzed both the cholesterol ester and the triacylglycerol portion of chylomicron remnants. 5. Three conclusions are drawn: (a) the study provides evidence against the existence of a plasma membrane-bound enzyme-hydrolyzing chylomicron cholesterol ester before or during its penetration into the cell; (b) enzymes of the cell sap and possibly of the endoplasmic reticulum can degrade cholesterol ester of chylomicron remnants without preceeding hydrolysis of the triacylglycerol core; and (c) lysosomal enzymes can degrade both the cholesterol ester and the triacylglycerol portion of chylomicron remnants if these are taken up as whole particles by endocytosis.
