Synthesis of lipoprotein lipase in the liver of newborn rats and localization of the enzyme by immunofluorescence.

In newborn rats, lipoprotein lipase (LPL) activity was higher in the liver than in several other tissues, such as heart, diaphragm or lungs, and accounted for about 3% of total LPL activity in the body. There was no significant correlation between LPL activity in liver and in plasma. Thus transport of the enzyme from extrahepatic tissues was probably not the major source of LPL in liver. To study LPL biosynthesis directly, newborn rats were injected intraperitoneally with [35S]methionine, and LPL was isolated by immunoprecipitation and separation by SDS/polyacrylamide-gel electrophoresis. Radioactivity in LPL increased with a similar time course in all tissues studied, including the liver. Substantial synthesis of LPL was also demonstrated in isolated perfused livers from newborn rats, whereas synthesis was low in livers from adult rats. There was strong LPL immunofluorescence in livers from newborn rats, mainly within sinusoids and along the walls of larger vessels. This labelling disappeared after perfusion with heparin, which indicates that much of the enzyme is in contact with blood and can take part in lipoprotein metabolism.