Hepatic apo-A-I and apo-E and intestinal apo-A-I are synthesized in precursor isoprotein forms by organ cultures of human fetal tissues.

In this report, we demonstrate that organ cultures of human fetal liver and intestine synthesize and secrete into the culture medium the major apolipoproteins. The liver cultures were shown to produce apo-A-I, apo-A11, apo-B, apo-C-11, apo-C-IIIs, and apo-E, and the cultures of intestine produced primarily apo-A-I. The synthesized apolipoproteins secreted into the culture medium were identified by direct two-dimensional gel analysis of the culture medium or by two-dimensional analysis following purification of the apoproteins by ultracentrifugation or immunoprecipitation. In both cases, two-dimensional gel electrophoresis revealed that apo-A-I produced by both liver and intestine consisted of two major isoproteins, designated 2 and 3, which are more basic than the major apo-A-I isoproteins which occur in plasma, designated 4 and 5. Similar analysis of the apo-E synthesized by the liver showed it to be mainly composed of an array of isoproteins with increasingly higher molecular weights and more acidic isoelectric points compared to the major apo-E isoproteins found in plasma. These synthesized apo-E isoproteins, presumably apo-E precursors, were converted to the major 38,000-dalton apo-E isoprotein(s) of plasma after treatment with Clostridium perfringens neuraminidase and represent sialo apo-E isoproteins. The hepatic apo-A-11, apo-C-11, apo-C-111-1, and apo-CIII-2 correspond to the apoprotein forms seen in plasma. These findings, combined with our recent studies of apo-A-I synthesis in organ cultures of adult human intestine (Zannis, V. I., Breslow, J. L., and Katz, A. J. (1980) J. Biol. Chem. 255, 861243617), are consistent with the hypothesis that apo-A-I and apo-E are synthesized and secreted in precursor isoprotein forms which are subsequently modified to attain the isoprotein forms present in plasma. The phenomenon of posttranslational apoprotein modification is an important but previously unrecognized step in the regulation of lipoprotein metabolism.