Egg White Avidin

Abstract

As judged by several physical-chemical criteria, egg white avidin has been purified as a homogeneous glycoprotein from a commercial preparation.The avidin subunit contains 129 amino acid residues, 4 residues of mannose, and 3 residues of glucosamine as determined by amino acid and carbohydrate analyses.Of four cyanogen bromide peptides purified by gel filtration, one was an l&residue peptide from the NH2 terminus, one was a 78-residue peptide from the middle of the peptide chain, one was a 32-residue peptide from the COOH terminus, and one was a 96-residue peptide containing all of the residues present in the first two peptides.This last peptide was evidently the result of the conversion of methionine in the sequence Met-Thr to homoserine without cleavage of the peptide bond.Inasmuch as these cyanogen bromide peptides account for 128 residues as compared to 129 found by analysis of the protein, it is evident that the subunits of avidin are identical.The sequences of the NH&erminal and COOH-terminal cyanogen bromide peptides have been determined.The carbohydrate is attached to Residue 17 of the protein in the sequence Asn(carbohydrate)-Met-Thr. This is another example of the general sequences Asn(carbohydrate)-X-Thr or Asn(carbohydrate)-X-Ser found for most glycoproteins in which the carbohydrate is attached to the protein through the side chain of an asparaginyl residue.It was recognized as early as 1898 that ingested, uncooked egg white was toxic (1, 2) and could produce in test animals a syndrome with many of the characteristic features of classical vitamin B deficiency (3).By 1940 it had become evident that the toxic effects of egg white were caused by a protein which bound an essential growth factor, thus preventing its absorption from the intestine (4-6) or from the surrounding medium in the case of microorganisms (7, 8).This growth factor had been called protective factor X (9), vitamin H (lo), the factor protective against egg white injury (ll), and coenzyme R ( 12), but