174 Organic Constituents of Saliva 



These classes do not account for the consistent presence of 

 mannose and glucose and occasionally ribose in hydrolysates of 

 mucin. Presumably there is at least one more chemical class of 

 mucin (Blix, 1940; Blix, Svennerholm and Werner, 1952; Werner, 

 1953; Kent and Whitehouse, 1955; Odin, 1958). 



There is no evidence that mucoitin sulphuric acid is a normal 

 constituent of saliva as neither isolated salivary mucin nor, saliva 

 contain appreciable amounts of ester sulphate (Blix, 1936; Meyer, 

 1945 ; Knox and Still, 1953). The high sulphur content reported in 

 samples of "isolated mucin" (for instance by Komarov and Stav- 

 raky, 1940) is due to the presence of sulphur containing amino- 

 acids. It is interesting that both the other mucoprotein classes are 

 characterized by a high proportion of sugars with the galactose 

 configuration (i.e. galactose, 6-deoxygalactose, galactosamine and 

 galacturonic acid). The sialomucins are quite labile and readily 

 lose sialic acid in mild acid solutions. They are probably a major 

 constituent of submaxillary saliva proteins but their presence in 

 parotid saliva in man or animals does not seem to have been 

 looked for. Bovine submaxillary mucin prepared by McCrea's 

 method contained 27-5 per cent sialic acid, 12 per cent galacto- 

 samine, 4 per cent glucosamine and 3-5 per cent galactose and 

 fucose (Odin, 1958). The glucosamine and fucose content is prob- 

 ably due to admixture of some fucomucins with the predominant 

 sialomucins. On the other hand a typical fucomucin from a pseudo- 

 mucinous ovarian cyst contained 19 per cent hexosamine, 10 per 

 cent galactose, 9 per cent fucose and i-6 per cent sialic acid. An 

 important fucomucin present in submaxillary saliva has blood 

 group properties. 



Sialic acid. Sialic acid was originally crystallized from a material 

 released from a mild acid hydrolysis of submaxillary mucin. In its 

 reactions it resembles N-acetyl-hexosamine but gives an Ehrlich 

 reaction without alkali pretreatment and a purple colour with 

 orcinol. This acid has been identified in many muco- and glyco- 

 proteins, including submaxillary mucin, nasal, tracheo-bronchial, 

 gastric and intestinal mucus, brain ganglioside, urinary mucopro- 

 tein, erythrocytes and amyloid substances (Klenk, 1941, 1958; 

 Werner, 1953; Zilliken, 1956; Zilliken and Whitehouse, 1958; 

 Gottschalk, i960). 



The structure of sialic acid seems now to be well established as 

 a substituted 9 carbon amino-ketoseuronic acid of a quite novel 



