Pl. Hurst et Cj. Clark, POSTHARVEST CHANGES IN AMMONIUM, AMINO-ACIDS AND ENZYMES OF AMINO-ACID-METABOLISM IN ASPARAGUS SPEAR TIPS, Journal of the Science of Food and Agriculture, 63(4), 1993, pp. 465-471
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.