The action of lecithin-cholesterol acyltransferase (LCAT, EC 2.3.1.43) on the different pig lipoprotein classes was investigated with emphasis on low-density lipoproteins (LDL). It was demonstrated previously that LDL can serve as substrate for LCAT, probably because they contain sufficient amounts of apoA-I and other non-apoB proteins, known as LCAT activators. Upon a 24-h incubation of pig plasma in vitro in the presence of active LCAT, both pig LDL subclasses, LDL-1 and LDL-2, fused together, forming one fraction, as revealed by analytical ultracentrifugation. This fusion was time dependent, becoming visible after 3 h and complete after 18 h of incubation. Concomitantly, free cholesterol and phospholipids decreased and cholesteryl esters increased. When isolated LDL-1 and LDL-2 were incubated with purified pig LCAT for 24 h, LDL-1 floated toward higher densities and LDL-2 toward lower densities, although this effect was not as pronounced as in incubations of whole serum. In further experiments, pig serum was incubated for various periods of time in the presence and absence of the LCAT inhibitor sodium iodoacetate. The individual lipoproteins then were separated by density gradient ultracentrifugation or by specific immunoprecipitation and chemically analyzed. Both methods revealed that in the absence of active LCAT there was a transfer of free cholesterol from LDL to high-density lipoproteins (HDL) and a small transfer of cholesteryl esters in the opposite direction. In the presence of LCAT the loss of free cholesterol started immediately in all three lipoprotein classes, was most prominent in LDL, and was proportional to the newly synthesized cholesteryl esters incorporated in each fraction.(ABSTRACT TRUNCATED AT 250 WORDS)