Metabolism of the Pectic Substances

Abstract

This thesis examines pectic metabolism by oat coleoptile sections. Pectin accounts for 5 per cent of the dry weight of such sections. Approximately 5 per cent of the pectin is cold water soluble--70 per cent alcohol insoluble. Approximately 50 per cent of the carboxyl groups of this fraction are methyl esterified. Hot water solubilizes from the cell wall a highly esterified pectin fraction which represents 15 per cent of the total. In the remaining hot water insoluble pectin, only 30 per cent of the pectic carboxyl groups are combined as esters. Pectins, once formed, are metabolized very slowly. There is little or no mixing of the various pectic fractions during a 15-hour period. The methyl ester group of pectin is supplied by the methyl group of methionine. Oat coleoptile sections, either intact or as homogenates, form S-methylmethionine and methionine sulfoxide from methionine. Both of these compounds are also active as methyl donors for the formation of pectic esters. Indoleacetic acid accelerates both incorporation of the methyl of methionine into methyl ester moieties and the incorporation of glucose into galacturonic acid residues of water soluble pectins. This increase is a measure of an accelerated rate of pectin synthesis. Indoleacetic acid does not increase the rate of synthesis of the water insoluble pectins. Evidence is presented which favors the supposition that the methyl ester groups are formed before polymerization of the galacturonic acid residues. In vitro incorporation of the methyl of methionine into methyl ester groups of pectin has not been achieved. Limited success has been obtained with the in vitro incorporation of glucose into galacturonic acid residues of water soluble pectins.