Regulation of Phenylalanine Ammonia‐Lyase Activity in Cell‐Suspension Cultures of Petroselinum hortense

The time courses for induced changes in the phenylalanine ammonia‐lyase activity at five different stages during the growth cycle of cell‐suspension cultures from parsley (Petroselinum: hortense Hoffm.) were investigated. Large increases in the enzyme activity, induced either by irradiation or by dilution of a cell culture into fresh medium, were followed by an exponential decline to the initial low level. The maximum inducible level of specific enzyme activity varied within a range of about six‐fold, depending on the mode of induction and the growth stage of the cell culture. The general shapes of the curves for the changes in enzyme activity were similar under the various experimental conditions. However, the precise positions of the peaks in the activity varied from about 12 – 27 h after the onset of induction. Proportionally large variations were found for the peak widths at half‐maximum and, in the case of induction by continuous irradiation, for the periods of time required for half‐maximal induction of the system with light. The apparent half‐life of the enzyme, as calculated from the rate of decline of the activity subsequent to the peak, remained approximately constant under all conditions investigated. A general model is proposed which would explain the regulation of phenylalanine ammonialyase activity in Petroselinum hortense cell cultures by an interplay of potentially large variations in the rate of synthesis and an approximately constant rate of degradation of the enzyme. This conclusion is supported both by the various experimental results and by theoretical derivations of curves for the apparent changes in the enzyme‐synthesizing activity and in the rates of provision of this activity at the site of enzyme synthesis.