Biosynthesis of protoheme and heme a from glutamate in maize

The heme and chlorophyll precursor delta-aminolevulinic acid (ALA) can be formed by two biosynthetic routes: from the intact carbon skeleton of glutamate via a five-carbon pathway, which occurs in chloroplasts and bluegreen algae, and by ALA synthase-catalyzed condensation of succinyl-CoA and glycine, which occurs in bacteria and animal mitochondria. The biosynthetic route of plant mitochondrial heme a was determined by incubating terminal epicotyl sections of 8-day-old etiolated Zea mays seedlings in the dark with l-1-[(14)C]glutamate (which can be incorporated into ALA only via the five-carbon route) or 2-[(14)C]glycine (which would be incorporated via ALA synthase). Label incorporation was measured in highly purified protoheme and heme a. In 12-hour incubations, label uptake was greater than 70%. Total cellular protoheme was labeled 29.7 times more effectively by glutamate than glycine. Heme a was labeled 4.1 times more effectively by glutamate than by glycine. To assess the relative ability of the two amino acids to contribute label to the farnesyl moiety of heme a, label incorporation into total cellular nonsaponifiable lipids was measured. Glycine labeled this fraction 11.3 times more effectively than glutamate. Thus, a contribution by glycine to the farnesyl moiety may account for the small amount of label appearing in heme a. Our results indicate that in etiolated maize, noncovalently bound hemes, including mitochondrial heme a, are made mostly, and possibly entirely, from ALA synthesized via the five-carbon pathway. There is little or no contribution from ALA formed via ALA synthase, and no evidence was found for the operation of this enzyme in maize.