Comparative study of heme and Mg-protoporphyrin (monomethyl ester) biosynthesis in isolated pea chloroplasts: effects of ATP and metal ions.

摘要

The flux of porphyrins through the iron and Mg-chelatase reactions in pea cv. Spring chloroplasts was estimated. The chloroplasts were administered exogenous delta-aminolevulinic acid (ALA) and haem synthesis was quantified by incorporation of [14C]ALA. Flux through Mg-chelatase was estimated by measuring levels of the first committed intermediates in the pathway, Mg-protoporphyrin-IX + its monomethyl ester (Mg-Proto(Me)). In the presence of ATP, four major effects were noticeable: Mg-Proto(Me) accumulation was ten-fold greater than haem accumulation; haem synthesis was suppressed by 68%; Mg-Proto(Me) accumulated, and no protoporphyrin-IX was detectable; the total amount of Mg-Proto(Me) + haem was 80% higher than the protoporphyrin + haem contentin the absence of ATP. The accumulation of protoporphyrin from ALA could be increased 30% by incubation with ATPgammaS; under these conditions no Mg-Proto(Me) accumulated. The presence of Fe2+ stimulated Mg-protoporphyrin-IX synthesis from both ALA and porphobilinogen substrates. This stimulation was observed at 0.1 but not at 1 mM ALA. Iron also stimulated the uptake of ALA into the chloroplasts; the time course of ALA uptake in the presence of Fe2+ resembled that for Mg-Proto(Me) synthesis at 0.1 mM ALA in the presence of Fe2+.