Light-induced phenylalanine ammonia-lyase in cell-suspension cultures of Petroselinum hortense: Quantitative comparison of rates of synthesis and degradation

Abstract

Immunological methods were used to determine in vivo the rates of synthesis and degradation of phenylalanine ammonia-lyase (EC 4.3.1.5) in irradiated cell-suspension cultures of parsley ( Petroselinum hortense Hoffm.). Irradiation of the cells with ultraviolet light induced an immediate, exponential increase in the rate of enzyme synthesis. In contrast to previous assumptions, this increase occurred with no detectable lag period. The increase continued at a high rate for several hours even in the dark period following short-term irradiation for 2.5 h. The maximally induced rate of enzyme synthesis in cells from a linear growth period was much larger than in cells from the stationary phase of the culture, both after short-term and continuous irradiation. The half-life of the enzyme, as determined by labeling studies in vivo, varied between about 10 h and 5 h for the two different growth stages. The curves for light-induced changes in enzyme activity coincided with corresponding curves calculated from the data obtained for the rates of enzyme synthesis and degradation. This coincidence supports the previous hypothesis that large and rapid changes in the rate of synthesis and an essentially constant rate of degradation determine the shapes of the curves for light-induced changes in phenylalanine ammonia-lyase activity at a given growth stage of a parsley cell culture.