STRUCTURE OF RABBIT HAPTOGLOBIN . PREPARATION AND PROPERTIES OF GLYCOPEPTIDES

Phylogenic research on the haptoglobin of different animal species led us to study the structure of rabbit haptoglobin. Rabbit haptoglobin, whose homogeneity was checked according to several criteria, was submitted to hydrolysis by pronase (72 h, 39°, pH 8.2); the hydrolysate was then filtered through Sephadex G‐25, and the resulting glycopeptide fraction was separated on Dowex 50‐X2 (H+) into two parts, one (A) not retained by the resin, the other one (B) collected after elution with 0.5 M NaCl. Amino‐acid and carbohydrate composition, nature of end groups, and timing of sialic acid release, were determined on fraction A; from the amount of aspartic acid, the molecular weight was estimated to be no less than 2860. Fraction B, which amounted to one third of fraction A, could not be analysed on account of the insufficient quantity available. Quantitative paper electrophoresis at pH 3.5 and 6.2 yielded 8 glycopeptides from fraction A and 2 from fraction B. These various glycopeptides showed notable differences in their amino‐acid composition, mainly with respect to glutamic acid, proline, glycine and histidine. On the contrary, no significant differences were found in the carbohydrate composition, with the exception of the oses N‐acetylneuraminic acid ratio, which varies from 2 to 10. These results suggest 6 oligosaccharide units to be present in rabbit haptoglobin, as compared to 7 in human haptoglobin, bound to peptidic fractions varying in their amino‐acid composition in the neighbourhood of the carbohydrate‐peptide linkage. Although the precise nature of this bond has not been ascertained, it does not appear to be an O‐glycosidic one. Copyright © 1969, Wiley Blackwell. All rights reserved