Abstract
It has been suggested that hepatic triglyceride lipase (H-TGL) plays a role in the hydrolysis of serum lipid. However, the precise function of the enzyme is not yet clear. Based on two lines of reasoning we emphasize the importance of direct measurement of the plasma membrane-bound HTGL activity for elucidating the function of the enzyme in the rat liver. First, when the H-TGL activity in post-heparin plasma is measured, it covers not only the enzyme activity originating in the liver, but also that from the ovaries and adrenal glands of the rats. Second, recent studies have indicated that the H-TGL in the liver is located mainly on the plasma membranes and is the origin of the lipase released from the liver by heparin.
Plasma membrane-associated H-TGL activity was reduced by fasting and streptozotoein diabetes, whereas is was partially restored by refeeding and insulin treatment, respectively. The H-TGL activity of rats fed a no-fat diet was higher than that of those fed a fat-containing diet. The changes in enzyme activities under all these conditions were found to coincide with those of the plasma IRI (immunoreactive insulin) levels. It is strongly suggested therefore that the H-TGL associated with plasma membranes is under hormonal regulation by insulin.
In the present study, no relationship was observed between the enzyme activity and serum triglyceride or cholesterol levels. The physiological significance of the enzyme in the hydrolysis of serum lipids remains obscure. However, it is suggested that H-TGL may function in order to deliver free fatty acids originating in the serum triglyceride for esterification in the liver and subsequent very low density lipoprotein synthesis in harmony with other enzymes affected by insulin.
