The role of hepatic triglyceride lipase in the metabolism of intermediate-density lipoprotein—Postheparin lipolytic activities determined by a sensitive, nonradioisotopic method in hyperlipidemic patients and normals

With a view to elucidating the physiologic role of hepatic triglyceride lipase (H-TGL), we studied the relationship between the activity of H-TGL and the concentrations of the lipids of ultracentrifugally separated lipoprotein fractions in sera from 81 cases of primary hyperlipidemia, 5 of hypothyroidism, and 31 normal subjects. The activity of H-TGL in postheparin plasma was determined by the sensitive, nonradioisotopic method that was recently developed by us. In the entire group of subjects including the normals, the activity of H-TGL had a significant inverse correlation with the concentration of the cholesterol (r = −0.443, P < 0.001) and phospholipid (r = −0.433, P < 0.001) of intermediate density lipoprotein (IDL). When the patients were divided into subgroups according to the phenotype of hyperlipidemia, it was found that the correlation was more significant in type IIb (r = 0.695, P < 0.001) or type IV + V (r = −0.664, P < 0.0001). In the five cases of hypothyroidism, the mean IDL cholesterol level was high (28.3 ± 12.3 mg/dL) and the H-TGL activity was very low (4.6 ± 4.5 μmol/h/mL). The H-TGL activity was also significantly correlated with the ratio of high density lipoprotein-2 to high density lipoprotein-3 cholesterol (r = 0.351, P < 0.001) in the entire group. The present study provided further evidence in favor of the hyptohesis that a decrease in H-TGL activity in hyperlipidemic patients with high levels of very low density lipoprotein (VLDL), such as type IIb, IV, and V cases, contributes to IDL accumulation, and suggests that H-TGL plays an important role in IDL metabolism, especially in hyperlipidemia associated with increased levels of VLDL.