On the existence of an unidentified sulphur grouping in the protein molecule. Part II.—On the estimation of cystine in certain proteins

In the preceding paper reasons were adduced for the belief that certain proteins contain a sulphur grouping other than cystine. It was shown that the uncertainty which exists with regard to the nature of the sulphur linkage is due in part to the difficulty experienced in isolating and estimating cystine. The present paper is concerned mainly with the determination of cystine and of “non-cystine” sulphur in ovalbumin. Perhaps the most striking difference between albumin derived from serum and that from egg lies in the behaviour of the combined sulphur. The percentage of total sulphur in these two proteins is not widely different. Osborne found 1·93 per cent. for serum albumin, and 1·62 per cent. for ovalbumin. But whereas in serum albumin the “loosely bound sulphur” is 66 per cent. of the whole, in ovalbumin it is less than half this amount. Moreover the maximum yield of cystine hitherto isolated from the hydrolysis products of ovalbumin is only about one-tenth of that from serum albumin. The form of combination of the preponderating fraction of the sulphur in ovalbumin still remains to be discovered.