G. R. TRISTRAM 



Before the two proteins are compared the distribution of the sulphur 

 acids in myoglobin must be discussed in some detail. J. Roche et afi 

 and A. Rossi-Fanelli 6 (see p. 115) both reported the presence of 

 cysteine equivalent to one molecule of SH in a protein molecule of 

 17,000. Rees (unpublished experiments) using an elaboration of the 

 method of sulphur distribution 14 could find no evidence for the presence 

 of cystine-cysteine and this was confirmed by the absence of any 

 nitroprusside test, even after treating the protein with cyanide. It is 

 probable that the protein used by the earlier workers was contaminated 

 by a cysteine-containing impurity since the cysteine content was, on 

 occasions, considerably lower than the minimum required for one 

 molecule 6 . 



DISCUSSION OF ANALYTICAL DATA 



The amount of any one amino-acid has little significance in the inter- 

 pretation of structure and biogenetic relationships 15 . 



Similarly, no significance can at present be attached to the ratio of 

 one amino acid to another unless there is definite evidence that the 

 amino acids or other groups concerned are functionally interrelated. 

 Thus Roche 5 and Rossi-Fanelli (p. 1 15) have compared the histidine/iron 

 ratios of various haemoglobins and also the sulphur/arginine ratios. Thus 

 while the former may be significant because of possible connections 

 between histidine and haem, the latter ratio is without meaning because 

 sulphur is not present in the form of a single substance but, in the 

 pigments of the various species, may be contained in varying mixtures 

 of cystine, cysteine and methionine. 



K. Bailey 15 has suggested that it is probably more valuable to 

 compare the distribution of the various polar and non-polar groups 

 (cf Table II). 



Comparison of the proteins — The amino acid data in Table I demon- 

 strate the basic nature of the two proteins and show that it is produced 

 by a significantly different mixture of amino acids. Thus in myoglobin 

 62 per cent of the basic groups are due to lysine and 31 per cent to 

 histidine whereas lysine and histidine each contribute 40 per cent of 

 the basic groups of haemoglobin. The total ionic groups of the proteins 

 are also significantly different. Haemoglobin contains 27 per cent of 

 ionic groups (cationic and free anionic) and is similar to other serum 

 proteins (cf Tristram 16 ). Myoglobin contains 34 per cent of such 

 groups and in this respect bears a distinct resemblance to the muscle 

 proteins myosin (34-1 per cent) and tropomyosin (45 per cent) (see 

 Bailey 15 ). 



Myoglobin as a precursor of haemoglobin — The simple relationship 

 between the molecular weights of myoglobin (17,000) and haemoglobin 



112 



