Modification of L-phenylalanine ammonia-lyase in soybean cell suspension cultures by 2-aminooxyacetate and L-2-aminooxy-3-phenylpropionate.

Suspension-cultured cells of soybean (Glycine max (L.) Merr. cv. Kanrich) produce large amounts of phenylalanine ammonia-lyase (PAL; EC 4.3.1.5), the first enzyme of phenylpropanoid metabolism, during growth. 2-Aminooxyacetic acid (AOA) and L-2-aminooxy-3-phenylpropionic acid (L-AOPP) inhibit the enzyme competitively in vitro and have been used for in vivo studies. The amount of extractable enzyme in the cells and their utilization of NO 3 (-) and NH 3 (+) are reduced upon the addition of AOA. When AOA was added at various times during growth, the appearance of additional enzyme activity was prevented but enzyme already formed was not inhibited. No evidence was obtained for the presence of an inhibitor in the extracts and AOA inhibition in vitro was readily reversible. It is conculded that AOA acts to inhibit the formation of PAL in suspension-cultured soy bean cells. In vitro inhibition of soybean PAL by L-AOPP could not be reversed; in contrast, the inhibition of maize (Zea mays L.) PAL was readily reversible. Added L-AOPP, which was rapidly taken up by the soybean cells, prevented the large increase in enzyme activity. Although PAL activity was blocked in the cultures, no appreciable increase in phenylalanine content could be detected in cell extracts. The response of soybean cell suspensions to L-AOPP addition thus differs from that of other tissues which in presence of L-AOPP show an increase in PAL activity and an accumulation of phenylalanine.