Quaternary structure of erythrocruorin from the nematode Ascaris suum. Evidence for unsaturated haem-binding sites

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Abstract
The quaternary structure of erythrocruorin from the nematode Ascaris suum was studied. The native protein had a sedimentation coefficient, at a protein concentration of 1 mg/ml, of 11.6 .+-. 0.3 S and an Mr, as determined by sedimentation equilibrium, of 332000 .+-. 17000. SDS/polyacrylamide-gel electrophoresis gave one band with a mobility corresponding to an Mr of 43000 .+-. 2000. The Mr of the polypeptide chain was determined to be 41600 .+-. 1500 by sedimentation equilibrium in 6 M-guanidinium chloride and 0.1 M-2-mercaptoethanol. Cross-linking with glutaraldehyde followed by SDS/polyacrylamide-gel electrophoresis yielded a maximal number of eight bands. The haem content of Ascaris erythrocruorin was observed to vary from one preparation to another. This finding was shown to be due to non-realization of the full binding capacity for haem. By titration with haemin, the haem content was found to attain a maximal value of 2.86 .+-. 0.14%, corresponding to a minimal Mr per haem group of 21 600 .+-. 1000. Our findings indicate that Ascaris suum erythrocruorin is composed of eight identical polypeptide chains, carrying two haem sites each.

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