1. Cultured eel-hepatocytes synthesized and secreted the only one kind of lipoprotein, namely VLDL. Although there are two main lipoproteins in eel serum, VLDL and HDL, eel-hepatocytes did not synthesize and secrete HDL. 2. Characterizations of the VLDL. (1) Chemical composition of the VLDL. The contents of rotein, triacylglycerol, free cholesterol, cholesterol ester and phospholipid in the VLDL were 12, 69, 3.6, 0.4 and 14%, respectively. (2) Apoproteins of the VLDL. Main apoproteins of the VLDL were apo A and B. What is most different from VLDLs secreted by mammalian livers is that the secreted VLDLs of mammals do not contain apo A as a main apoprotein. (3) The rate of the VLDL secretion.
Cultured eel-hepatocytes secreted the VLDL at the rate of 3.79*1.51mug VLDL/mg cell protein/h(n=6). This value was 10 times higher than the rates of VLDL secretion by human hepatocytes. 3. Effects of insulin and serum lipoproteins. (1) Effect of insulin. Insulin inhibited the secretion of the VLDL and the incorporation of 14C-leucine and 14C-acetate into the VLDL. (2) Effect of serum VLDL. When serum VLDL was added to the eel-hepatocytes, the hepatocytes stimulated the VLDL secretion. Furthermore, the synthesis of apoproteins of the VLDL, especially apo B, was stimulated by the serum VLDL. 4. When the secreted VLDL was incubated with liposomes or lipoprotein lipase, a HDL-like particle was formed. 5. The presence of high content of lipid in eel muscle was histochemically observed. When the incorporation of 14C-VLDL into the muscle, liver, heart and kidney was investigated after the 14C-VLDL was injected into the portal vein, the incorporation into the muscle was the highest. Furthermore, it was found that there was the protein on the plasma membrane of eel muscle which could bind the VLDL specifically and of which molecular weight was 77,000.