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The rate of release of amino acids during partial acid hydrolysis of bovine submaxillary mucin has been studied. A number of di- and tripeptides have been isolated on the amino acid analyzer. Three β-hydroxyamino acid peptides, Thr-Thr-Thr, Ser-Ser, and Thr-Ser have been isolated and account for 12 % of these amino acids in native mucin. A substantial amount of the glycine and alanine has been isolated as dipeptides of serine and threonine with the fihydroxyamino acids in the amino-terminal position. Proline, valine, isoleucine, and leucine seem to be in close association in the peptide core. The same pattern of peptides has been obtained from the partial acid hydrolysis of ovine submaxillary mucin.

A secretory glycoprotein was isolated from the hamster sublingual gland. It contains 65% carbohydrates, the remainder being protein. The principal type of sialic acid was identified as N-acetylneuraminic acid with about 1/4 of the molecules O-acetylated. The hexosamine occurs mainly as N-acetylgalactosamine; other sugars present are galactose and fucose. The carbohydrate side chains are linked to the protein core by an O-glycosyl linkage between seryl and threonyl residues and N-acetylgalactosamine. The glycoprotein has an apparent molecular weight of 330,000. The sialogly coprotein may act as a biological antifreeze. Relevant differences in the chemical composition of the secretory glycoproteins of the sublingual and submaxillary salivary glands of this species are briefly discussed.

Publisher Summary This chapter discusses the nature of the protein core of bovine, ovine, and porcine submaxillary mucins and describes its implications for protein synthesis. Protein biosynthesis occurs by a two-step mechanism involving the formation of small peptides at the ribosomes and their assembly by polymerases at the Golgi membrane. The existence of homologous peptides and proteins in the mucins from individual glands has important consequences for the concept of evolutionary processes. The lack of blood group activity for the inactive mucin can be expected to arise from the presence of carbohydrate chains shorter than for those for the H and A types. A limited number of synthesizing sites for peptides, containing 16 to 30 amino acids, can produce a pool of peptides from which an unlimited number of proteins can be assembled at a specific enzyme site for each protein. Specific short sequences needed for a particular function—for example, the basic chains of histones—can be separately synthesized and incorporated.

Bovine submaxillary mucin (a glycoprotein) was hydrolyzed for varous times (1 to 300 hr) at 110° in 6 N HCl (0.3 mg protein per ml). The kinetics of destruction of serine were found to be first-order and that of threonine to be zero-order. The velocity constant for the destruction of serine was k – 4.3 X 10-3 hr-1 and of threonine was k – 5.6 X 10-4 mole hr1. The amounts of serine and threonine destroyed after 22 hours hydrolysis were 10.0 % and 2.7 %, respectively. Since the losses were the same in the absence of carbohydrates and since several prior extensive investigations reported similar corrections for serine, these corrections may be applicable to most proteins and glycoproteins.