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Activity of 5-aminolevulinic acid (ALA) dehydratase (ALAD, (EC 4.2.1.24)) and soluble protein content were determined in 'Rutgers' tomato (Lycopersicon esculentum Mill.) fruit pericarp extracts during development and ripening. ALAD activity in several organs of tomato plant also was determined. Fruit tissue was analyzed at 5-day intervals between days 10 and 60 postanthesis. ALAD activity in fruit tissue declined over time, with the most pronounced decrease occurring between days 10 and 25. At the mature green stage (day 40), and before the breaker stage (day 45), activity of ALAD had declined to a steady-state minimum, and it remained detectable at residual levels throughout ripening (days 40 to 60). Soluble protein content declined less rapidly than did ALAD activity. Immunoblot analysis showed that ALAD protein existed as a doublet of isozymes. One isozyme decreased in abundance, whereas the other isozyme remained constant during development and ripening. ALAD activity was greatest in extracts of chlorophyllou s organs (stems, leaves, immature fruit) but only marginally detectable in extracts of nonchlorophyllous organs (roots, overripe fruit). The pH optimum and Km for tomato fruit ALAD were similar to those of ALAD isolated from other sources. Abbreviations: ALA, 5-aminolevulinic acid; ALAD, ALA dehydratase; PBG, porphobilinogen; SDS-PAGE, sodium dodecyl sulfate-polyacry - lamide gel electrophoresis. One of the first committed enzymes in the chlorophyll biosyn- thetic pathway is 5-aminolevulinic acid (ALA) dehydratase (ALAD, (EC 4.2. 1.24)). ALAD catalyzes the condensation of two ALA molecules into the monopyrrole porphobilinogen (PBG) (Jordan and Seehra, 1980). Light regulation of several chlorophyll biosyn- thesis enzymes, including ALAD, has been documented (Castelfranco and Beale, 1983; Harel, 1978). Previous studies with several plant species have shown that ALAD protein content and enzymatic activity depend upon the type of tissue and the stage of development during light-induced greening. ALAD activity in- creased during light-induced development of cotyledons of mus- tard (Sinapis alba L.) seedlings, and phytochrome involvement in ALAD light induction was established (Kasemir and Masoner, 1975). Huault et al. (1987) found parallel increases in ALAD protein content and enzyme activity during chloroplast differentia- tion in etiolated cotyledons of radish (Raphanus sativus L.). Steady-state levels of ALAD mRNA exhibited little or no change during light-induced greening of etiolated pea (Pisum sativum L.) leaf tissues, and this suggested postranscriptional light regulation of ALAD activity, either at the level of translation or enzyme activation (Boese et al., 1991). Little research has been conducted on ALAD activity in ripen- ing or senescing tissues, but Naito et al. ( 1980) showed that ALAD activity decreased continuously with age of primary leaves of bean (Phaseolus vulgaris L.). The purpose of this research was to

Activity of 5-Aminolevulinic Acid Dehydratase Declines during Tomato Fruit Development and Ripening