J. Biol. Chem., Vol. 261, Issue 15, 6658-6669, 05, 1986
Biosynthesis of proteoglycans by rat embryo parietal yolk sacs in organ culture
RV Iozzo and CC Clark
The embryonic rat parietal yolk sac has been previously shown to synthesize
a number of basement membrane glycoconjugates including type IV
procollagen, laminin, and entactin. In this study, parietal yolk sacs were
isolated from 14.5-day rat embryos and incubated in organ culture for 4-7 h
with [35S]sulfate, [3H] glucosamine, and/or 3H- labeled amino acids, and
the newly synthesized proteoglycans were characterized. The major
[35S]sulfate-labeled macromolecule represented approximately 90% of the
medium and 80% of the tissue radioactivity. It also represented nearly 80%
of the total [3H]glucosamine-labeled glycosaminoglycans. After purification
by sequential ion-exchange chromatography and isopycnic CsCI density
gradient ultracentrifugation, size-exclusion high-performance liquid
chromatography showed a single species with an estimated Mr of 8-9 X 10(5).
The intact proteoglycan did not form aggregates in the presence of
exogenous hyaluronic acid or cartilage aggregates. Alkaline borohydride
treatment released glycosaminoglycan chains with Mr of 2.0 X 10(4) which
were susceptible to chondroitinase AC II and chondroitinase ABC digestion.
Analysis by high-performance liquid chromatography of the disaccharides
generated by chondroitinase ABC digestion revealed that chondroitin
6-sulfate was the predominant isomer. The uronic acid content of the
glycosaminoglycans was 92% glucuronic acid and 8% iduronic acid, and the
hexosamine content was 96% galactosamine and 4% glucosamine. No significant
amounts of N- or O-linked oligosaccharides were detected. Deglycosylation
of the proteoglycan with chondroitinase ABC in the presence of protease
inhibitors revealed a protein core with an estimated Mr of 1.25-1.35 X
10(5). These results indicated that the major proteoglycan synthesized by
the 14.5-day rat embryo parietal yolk sac is a high-density chondroitin
sulfate containing small amounts of copolymeric dermatan sulfate.
Hyaluronic acid and minor amounts of heparan sulfate proteoglycan were also
detected.