Lupin asparagine synthetase

Figure 2. Effect of 1,4-cineole (circles), cis-2-hydroxy-1,4-cineole (triangles) and the commercial herbicide cinmethylin (squares) on the activity of asparagine synthetase from lupin. The dotted line represents 50% inhibition of enzyme activity.

The specific activity of these enzymes in the plant fraction of lupin nodules increases over the same time period as the increase in Ne-fixing activity in the bacteroids (Scott et al., 1976 Bolandet ai, 1979 Reynolds and Farnden, 1979). Asparagine synthetase has only been reported to date in nodules of lupin (Scott et ai, 1976 Radyukina et ai, 1977 Boland et al., 1979), however, numerous reports of aspartate aminotransferase activity in the plant fraction of several legume species have appeared (Table I). 

Route c involves the conversion of aspartate to asparagine by glutamine-dependent asparagine synthetase (EC 6.3.5.4). The enzyme has had a checkered career and it has proved extremely difficult to prepare extracts with high activity. Joy and Ireland (1990) have described in detail suitable assay methods and discussed possible factors that may prevent the determination of maximum rates of activity. A heat-stable, dialyzable inhibitor was found in pea leaf homogenates that inhibited the lupin cotyledon enzyme (Joy et al. 1983). The presence of this inhibitor may well account for the current lack of detection of asparagine synthetase in green leaf tissue. 

The cotyledons of germinating seeds have proved to be a major source of asparagine synthetase activity and the enzyme has been studied in lupins (Rognes, 1975,1980 Lea and Fowden, 1975X soybean (Streeter, 1973), cotton... 

Tracer experiments in etiolated shoots of lupin and investigations of the level of p-cyanoalanine synthetase in seedlings of the same plant indicate, however, that the major part of asparagine in this plant is derived from aspartic acid. 

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