The mechanism responsible for the accumulation of chylomicrons in mixed hyperlipoproteinemia (Type 5) is not known. Assays of the plasma-clearing factor, lipoprotein lipase, with artificially prepared substrates have been normal or only mildly depressed in this condition. The recent report of a patient in whom there was marked depression of activity against native chylomicrons despite normal activity against an artificial substrate suggested, however, that there might exist a qualitative defect in the function of lipoprotein lipase undisclosed by assays with the artificial substrate in general use. Accordingly, the lipolytic activity of post-heparin plasma from five diabetic patients with mixed hyperlipoproteinemia was measured against two artificial substrates and two preparations of human chylomicrons. There was good correlation between activity against coconut oil and each of the three other substrates over a wide range of plasma glyceride concentrations and post-heparin lipolytic activities. The data support the conclusion that plasma post-heparin lipolytic activity in diabetic patients with mixed hyperlipoproteinemia is qualitatively normal.

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