Secretion of apolipoproteins in the suckling rat. Absence of low-molecular-weight apolipoprotein B of hepatic origin.

Synthesis of apolipoprotein B in the intestine and liver in suckling rats whose duodena had been infused with [3H]lysine was estimated by measuring the radioactivity in the mesenteric lymph and blood serum. Apolipoprotein B synthesized in the suckling rat intestine was an exclusively low-molecular-weight form. In the serum d less than 1.21 g/ml fraction obtained from the lymph-diverted rats, apolipoprotein B of high molecular weight was only labeled with [3H]lysine in suckling rats, whereas both forms of apolipoprotein B were labeled in the adult rats. These results suggest developmental changes in hepatic apolipoprotein B synthesis and secretion.