Complexes of rat alpha 1-macroglobulin and subtilisin are endocytosed by parenchymal liver cells

Rat alpha 1-macroglobulin was isolated from plasma. Gel electrophoresis of the denatured and reduced protein showed two bands, with Mr values of 163 000 and 37 000. The large subunit contained an autolytic site. This subunit was also split after reaction of the macroglobulin with trypsin. Electron microscopy showed that the macroglobulin changed towards a more compact conformation after reaction with this proteinase. Subtilisin, or alpha 1-macroglobulin, was labelled with a sucrose-containing radio-iodinated group that stays in lysosomes after endocytosis and breakdown of the tagged protein. After intravenous injection into rats, alpha 1-macroglobulin was cleared from plasma with first-order kinetics, showing a half-life of about 9 h, whereas complexes of alpha 1-macroglobulin and subtilisin were cleared with half-lives of only 3 min. Liver contained about 60% of the label at 30 min after injection of complexes. About 90% of the liver radioactivity was found in parenchymal cells isolated after perfusion of the liver with a collagenase solution. Subcellular fractionation indicated a lysosomal localization of the complexes. We conclude that endocytosis by parenchymal liver cells is the major cause of the rapid clearance of alpha 1-macroglobulin-proteinase complexes from plasma.