Polymorphism of collagen type III in rat arterial wall analyzed by combination of SDS-polyacrylamide gel electrophoresis (SDS-PAGE) with immunoblotting was described. SDS-PAGE was carried out with the gel containing 4M urea and a negatively charged reducer, thioglycolic acid was added into the running buffer of cathodic reservoir in the middle of electrophoresis to cleave disulfide bonds in collagen type III. The polymorphic collagen type III, the trimer without cross-linckings ([α
1(III)]
3), the dimer with cross-linkings ([α
*1(III)°]
2) and the monomer without inter-chain disulfide bonds (α
*1(III)) were identified and quantified by immunoblotting method, and the alteration in the polymorphism of collagen type III with aging was examined using the aorta, the brachial artery and the skin. The total quantity of collagen type III decreased with aging in the aorta, the brachial artery and the skin. In the aorta and the brachial artery, [α
*1(III)°]
2 was a major component, but in the skin it was a minor component and [α
1(III)]
3 was a major one. With aging, the ratio of [α
*1(III)°]
2 to [α
1(III)]
3 increased markedly in the aorta.
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