POSTHARVEST CHANGES IN AMMONIUM, AMINO-ACIDS AND ENZYMES OF AMINO-ACID-METABOLISM IN ASPARAGUS SPEAR TIPS

Harvested asparagus (Asparagus officinalis L) spears accumulate ammoni um and amino acids in their tips during storage at ambient temperature s. To further investigate these phenomena the authors held spears at 2 0-degrees-C in the dark after harvest and examined several parameters of amino acid metabolism in the tips. Soluble protein content declined faster than total protein over the 5-day storage period. Protein loss was accompanied by an increase in total free amino acids with asparag ine showing the most dramatic increase. A pronounced accumulation of a mmonium occurred, starting after 3 days. The activities of glutamine s ynthetase (EC 6.3.1.2) and aspartate and alanine aminotransferases (EC 2.6.1.1 and EC 2.6.1.2, respectively) declined slightly whereas gluta mate dehydrogenase (EC 1.4.1.2) activity almost doubled over 5 days. P henylalanine ammonia lyase (EC 4.3.1.5) activity declined rapidly duri ng the first 2 days and then partially recovered. Asparagine synthetas e (EC 6.3.1.1), asparagine aminotransferase (EC 2.6.1.14), asparaginas e (EC 3.5.1.1) and glutaminase (EC 3.5.1.2) activities were not detect able at any time during the storage period. Ammonium accumulation coul d be caused by increased glutamate dehydrogenase activity coupled with a shortage of aspartate.