CHANGES IN THE ACTIVITIES OF GLUTAMINE SYNTHETASE AND GLUTAMATE DEHYDROGENASE DURING LOW TEMPERATURE STORAGE OF ASPARAGUS SPEARS AND SUBSEQUENT SENESCENCE AT 25°C

Incorporation of ammonia into amino acids occurs mainly via the GS/glutamate synthase (GOGAT) – catalyzed reactions (Miflin et al., 1980). Glutamine synthetase (GS, EC 6.3.1.2) catalyses the first step in amino acid biosynthesis from ammonia. Glutamate dehydrogenase (GDH, EC 1.4.1.2) has long been accepted as a major enzyme not only in amino acid biosynthesis but also in the metabolic connection between the tricarboxylic acid cycle and amino acids/proteins. It also acts as an alternative to the glutamate synthase cycle under conditions of high ammonia or stress or it could act in the deamination of glutamate connecting the GDH with carbon metabolism rather than with nitrogen metabolism (Robinson et al., 1991).

Freshly harvested asparagus (Asparagus officinalis) spears are highly perishable. They deteriorate rapidly within 2-5 days with physiological changes including increased respiration rate, reduced soluble carbohydrate, protein and amino acids (Lill et al., 1990). This study has been undertaken to examine the activities of GS and GDH enzymes in the top and bottom portions of the spears and their amino acid content during storage at 1°C and after transfer to 25°C.