THE SOLUBLE NITROGEN FRACTIONS OF POTATO TUBERS; THE AMIDES

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Abstract
Various methods of extracting the soluble N fractions from potato tubers and of removing protein from them have been critically examined to determine their suitability for the analysis or quantitative isolation of the amide fractions. Limitations with the use of alcoholic extracts are set, not only by the sparing solubility of glutamine, but also by the effect on the analysis of amides by hydrolysis if alcohol is not removed by distillation under reduced pressure. Conditions are prescribed for the detn. of ammortia-N, easily hydrolysable amide-N, aspargine-amide-N and true amino-N. In terms of these fractions, a complete account can be given of the N content of alcoholic extracts and also of aqueous extracts which have been freed from protein by heat and subjected to further purification. Aqueous extracts, however, contain other complex, probably basic, substances which disappear on purification and which are absent from alcoholic extracts. The procedure for preparing crystalline amides from potato tuber was perfected in the light of the determined effect of various precipitants on the soluble N fractions. These procedures were Pb acetate followed by mercuric nitrate or by the Neuberg and Kerb reagent (mercuric acetate) and phosphotungstic acid applied to the nitrogenous compounds liberated from the mercuric precipitates. The method chosen utilized mercuric nitrate precipitation applied to the nitrate from lead acetate and the amides were fractionally crystallized from the mercury-free solns.[long dash]first from the aqueous then from alcoholic solutions. Two batches of 2-5 and 3-5 kgm. were worked up for the amides they contained. 71.5% of the easily hydrolyzable amide of the fresh tissue was recovered in the crystalline products. This represented 98% of the amide in the mercuric nitrate precipitates, which in one case contained 93% of the easily hydrolyzable amide of the fresh tissue. By these methods 86% of the easily hydrolyzable amide should be obtainable in the crystalline products. Tyrosine, glutamine and asparagine were obtained in pure form. The yields corresponded to 0.116 gm. tyrosine, 1.42 gm. asparagine and 1.51 gm. glutamine per 1000 gm. of fresh tuber tissue. The method thus makes possible the isolation of glutamine in high yield even in the presence of asparagine in greater quantity. The identification of the easily hydrolyzable amide of potato as glutamine leads to a reconsideration of the key position of this substance in the metabolism of potato. This is discussed in relation to current knowledge of the activity of amidases, and the known reactions of glutamic acid in the transamination reaction. This knowledge is consistent with the view that, under the conditions conducive to protein synthesis and salt-uptake in the cells of the potato tuber, the stored soluble nitrogen passes through glutamine enroute to protein. Thus glutamine functions as a particularly labile source of the N for protein synthesis.