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

Abstract

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 chlorophyllous 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.