The Enzymatic Synthesis Of 5aminolevulinic Acid

5-Aminolevulinic acid (ALA) is formed by two different biosynthetic pathways (Figure 2). One, found in plants, algae, and most bacteria, originates from glutamate, with glutamyl-tRNA and glutamate 1-semialdehyde as intermediates (18), and is traditionally referred to as the C5 pathway. The other pathway, found in mammals, fungi, and some photosynthetic bacteria, involves a single enzymatic step catalyzed by 5-aminolevulinic acid synthase (ALAS) (17). This latter route, often referred to as the Shemin, or C4, pathway, involves condensation between glycine and succinyl-CoA in a reaction in which the carboxyl group of glycine is lost by decarboxylation.

ALAS is the rate-determining step in mammalian and fungal heme synthesis, and intracellular levels of the enzyme are tightly regulated. Two enzymes exist in mammalian systems; a ubiquitous enzyme, ALAS1, which is encoded on chromosome 13 and which is subject to tight control in all tissues, and the erythroid enzyme, ALAS2, which is encoded on the X-chro-mosome and expressed constitutively in developing erythrocytes (9). The photosyn-thetic bacterium, R. spheroides, used for the isolation of the enzyme also has two genes, hemA and hemT(23).

Aminolevulinic acid can be synthesized using purified ALAS and the procedure can be adapted to prepare isotopically labeled ALA for enzyme synthesis of labeled later pathway intermediates. The ease of using

Figure 2. The biosynthesis of ALA. (a) ALA can be synthesized from glutamate by the C-5 pathway or (b) from glycine and suc-cinyl-CoA by the Shemin route. In the case of the latter, it is known that the proR-hydrogen of glycine is removed in the overall transformation into ALA.

ALAS has been greatly enhanced by the availability of the recombinant enzyme from R. spheroides arising from the cloned and overexpressed hemA gene (4).

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