Light-stimulation of alanine aminotransferase activity in dark-grown leaves of Lolium temulentum L. as related to chlorophyll formation

Abstract

The activity of alanine aminotransferase (=glutamate-pyruvate transaminase, GPT) in dark-grown first leaves of Lolium temulentum L. was increased, after an initial lag-phase of 4-6 hr, by more than 130% during the first 24 hr of light-exposure. In comparison, aspartate aminotransferase (=glutamateoxalacetate transaminase, GOT) activity rose by only 18%. Red light treatments of up to 60 min duration produced subsequent increases in GPT activity but the effects were too small to indicate a phytochrome-mediated response. The amounts of enzyme formed were equivalent to those obtained with similar incident intensities of white light. Retuern to darkness after light exposure resulted in an arrestation of the light-stimulated GPT increase. Pre-treatment with cycloheximide caused either stimulatory or inhibitory effects depending upon the concentration applied but, in general, chlorophyll formation and GPT activity responded in a similar manner, whilst GOT showed virtually no response. Chloramphenicol at 6x10(-3) M depressed chlorophyll and Fraction 1 protein synthesis but stimulated GPT activity.The data are discussed in relation to the possible roles of GPT in the leaf. It is suggested that the enzyme, as determined, may be a complex of forms and that at least part of the activity may be involved in the early stages of chlorophyll biosynthesis.