Studies on biosynthesis of alpha 2-antiplasmin in rat liver cells

Biosynthesis, intracellular processing and secretion of alpha 2-antiplasmin (AP) were studied in primary cultures of rat hepatocytes by the pulse-chase technique. The experiments demonstrated that AP is present intracellularly as a 73 kD form and is secreted as a 79 kD form with affinity for human plasminogen kringles. Addition of human plasmin to the culture medium resulted in the formation of a 141 kD protein, probably a plasmin-AP complex, concomitantly with the disappearance of the 79 kD protein. The presence of tunicamycin, a blocker of N-linked glycosylation in the endoplasmic reticulum, during culture resulted in expression of a 63 kD form in medium and cell lysates. This latter form was unchanged after reduction and displayed affinity for human plasminogen kringles. The 79 kD form in the culture medium was resistant to treatment with endo-beta-N-acetylglucosaminidase-H while the intracellular 73 kD form was degraded to 63 kD, indicating glycosylation in the Golgi complex. It is concluded that rat AP is synthesized in a 63 kD form which is glycosylated in the endoplasmic reticulum to a 73 kD form; and that this in turn is trimmed and glycosylated further in the Golgi complex to the 79 kD form, which is secreted. The glycosylation is apparently not necessary for the secretion or for the affinity for human plasminogen kringles.