General Properties of Saliva Proteins 175 



type. Cornforth, Daines and Gottschalk (1957) have obtained this 

 substance in low yield on heating N-acetyl-glucosamine and oxa- 

 lacetic acid ; further, on treatment of sialic acid with Vibrio cholerae 

 extracts, it is enzymatically split into N-acetyl-D-glucosamine and 

 pyruvate (Heimer and Meyer, 1956), so that sialic acid can be 

 regarded as an aldol condensate of these two substances. The name 

 agreed upon by most of the workers in the field is neuraminic 

 acid for the unsubstituted acid (R x = R 2 = H); the substituents 



CHOH 



/ \ 

 H 2 C CHNHR t 



OH I I 



V I 



)C CH— CH— CHOH— CHoOH 



COOH ° ORo 



Neuraminic acid 



in Rj and R 2 vary. Human and sheep sialic acids have an N-acetyl 

 substituent, pig an N-glycolyl and cow and horse 0,N-diacetyl 

 substituents ; the latter are not identical but differ in melting point, 

 optical activity and X-ray spectra, presumably they are stereoiso- 

 mers (Blix, Lindberg, Odin and Werner, 1956; Svennerholm, 



1956). 



Since the neuraminic acids are strongly reducing and the mucins 

 are not, glycoside formation between neuraminic acid and other 

 sugar residues must occur through the 2-keto group. On mild 

 hydrolysis of mucin in hot water or enzymatic hydrolysis with 

 V. cholorae extracts ("Receptor destroying enzyme" or RDE) or 

 influenza virus (Gottschalk, 1957) the only soluble material re- 

 leased may be neuraminic acid in amounts up to 50 per cent of 

 the total present in the unhydrolysed mucin or roughly 10 per 

 cent by weight of submaxillary mucin. It seems likely therefore 

 that much of the neuraminic acid in mucin is terminal. With more 

 vigorous hydrolysis some of the neuraminic acid is recovered as 

 the glycoside neuraminyl-N-acetyl-galactosamine (Gottschalk, 

 1958). It is possible that the major part of the carbohydrate of 

 mucin consists of these short disaccharide units covalently bound 

 to peptide side chains. The nature of the bond between glycosides 

 and amino-acids is at present quite obscure. The presence of neur- 

 aminic acid in a mucoprotein gives it strong acidic properties since 

 the pK' of neuraminic acid is about 27. The major component 



