Postharvest changes in ammonium, amino acids and enzymes of amino acid metabolism in asparagus spear tips

Abstract

Abstract Harvested asparagus ( Asparagus officinalis L) spears accumulate ammonium and amino acids in their tips during storage at ambient temperatures. To further investigate these phenomena the authors held spears at 20°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 asparagine showing the most dramatic increase. A pronounced accumulation of ammonium occurred, starting after 3 days. The activities of glutamine synthetase (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 glutamate dehydrogenase (EC 1.4.1.2) activity almost doubled over 5 days. Phenylalanine ammonia lyase (EC 4.3.1.5) activity declined rapidly during the first 2 days and then partially recovered. Asparagine synthetase (EC 6.3.1.1), asparagine aminotransferase (EC 2.6.1.14), asparaginase (EC 3.5.1.1) and glutaminase (EC 3.5.1.2) activities were not detectable at any time during the storage period. Ammonium accumulation could be caused by increased glutamate dehydrogenase activity coupled with a shortage of aspartate.