Enzymatic iodination of salivary proteins by the ¹²⁵l‐lactoperoxidase system

Abstract

– Purified milk lactoperoxidase and endogenous human salivary peroxidase were used to label the proteins of whole mouth saliva with [125I]iodide. The proteins were then analyzed by isoelectric focusing or they were subjected so one‐dimensional polyacrylamide gel electrophoresis at pH 8.4. The radioactivity of the resolved protein fractions was determined. There were three to four major and four to five minor areas of radioactivity which were carried together with more or less distinctive protein fractions. Amylase and albumin were shown to be the most effective in binding [125I]iodide. No significant differences were observed in the iodination patterns of salivary proteins iodinated in the presence of endogenous saliva peroxidase and those iodinated in the presence of added milk lactoperoxidase. Hydrogen peroxide was necessary for iodination to take place. The significance of iodoproteins and the role of salivary peroxidases in the nonthyroidal metabolism of iodine are discussed.

Enzymically iodinated human salivary proteins. Fractionation and characterization by column chromatography and electrofocusing

“…Most of the proteins of human parotid saliva with pl 6-7 seem to possess saccharolytic activity, as shown by Pronk (1976). That they can bind a considerable amount of iodine is in agreement with our previous results obtained by polyacrylamide-gel electrophoresis (Tenovuo & Sarimo, 1977).…”

“…Confirming our previous observations (Tenovuo & Sarimo, 1977) and in agreement with the results of Marchalonis (1969), the enzymic iodination of salivary proteins occurred only when both H202 and peroxidase were present in reaction mixtures. The incorporation of [1251]iodide into enzyme protein alone was negligible under the conditions used.…”

“…Milk lactoperoxidase and endogenous salivary lactoperoxidase have, however, very similar properties with respect to their ability to iodinate tyrosine and proteins and to inhibit the growth of cariogenic Streptococcus mutans (Tenovuo, 1976). The fractionation patterns of [1251]iodinated salivary proteins produced by these two peroxidases showed no significant differences when analysed by polyacrylamide-gel electrophoresis (Tenovuo & Sarimo, 1977). Therefore it should be acceptable that our iodination reaction mixtures in vitro were enriched with commercial milk peroxidase to increase the endogenous iodination capacity of the system.…”

“…It is also well documented that human whole-mouth saliva contains all the necessary components for the enzymic iodination ofits proteins in vivo, although the oral conditions may not always be the most favourable for this process to take place (Tenovuo, 1976). In our previous studies we showed that when endogenous salivary peroxidase or commercial milk lactoperoxidase were used in vitro, only a few salivaryprotein fractions contained a considerable amount of covalently bound 125I, as measured by one-dimensional polyacrylamide-gel electrophoresis (Tenovuo & Sarimo, 1977).…”