Biosynthesis, assembly and secretion of fibrinogen in cultured rat hepatocytes.
The biosynthesis, assembly and secretion of fibrinogen were investigated in cultured rat hepatocytes which were incubated with [35S]methionine. When initial rates of the synthesis of three fibrinogen subunits were compared, the A alpha-subunit was found to be synthesized significantly slower than the B beta- and gamma-subunits. Pulse-chase experiments revealed that the secreted fibrinogen contained different proportions of the newly synthesized subunits, depending upon the chase times. Radioactivity in the A alpha subunit, which initially had the highest level of the three, was rapidly decreased in parallel with the chase time. The gamma-subunit had an increasing amount of the radioactivity in the secreted molecule during the chase periods, whereas that in the B beta-subunit was gradually decreased at the later stages of chase. Analysis of intracellular components of fibrinogen confirmed that the nascent A alpha-subunit was most rapidly exhausted, and the gamma-subunit occupied the largest proportion among the non-assembled subunits at later stages of chase. Taken together, these results suggest that the synthesis of A alpha-subunit, which has the lowest rate, could be the rate-limiting step in the production and secretion of fibrinogen in cultured rat hepatocytes, in contrast with what has been proposed for human and rabbit fibrinogen, namely that the synthesis of B beta-subunit is the rate-limiting step. The results also indicate that there is a large intracellular pool of gamma-subunit.
Hirose
S
S
Department of Biochemistry, Fukuoka University School of Medicine, Japan.
Oda
K
K
Ikehara
Y
Y
D000818:Animals; D002478:Cells, Cultured; D004591:Electrophoresis, Polyacrylamide Gel; D005340:Fibrinogen / Q000096:biosynthesis / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D008715:Methionine / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013462:Sulfur Radioisotopes
D013462:Sulfur Radioisotopes; D005340:Fibrinogen; D008715:Methionine