Biosynthesis of porphyrins and heme from gamma, delta-dioxovalerate by intact hepatocytes.

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Abstract
The ability of hepatocytes to synthesize porphyrins and heme from .delta.-aminolevulinic acid derived from .gamma.,.delta.-dioxovalerate and alanine was assessed. An alternate pathway for .delta.-aminolevulinic acid synthesis, in contrast to the condensation of succinate and glycine by .delta.-aminolevulinate synthase (EC 2.3.1.37), has been suggested. This has been supported by the isolation of .gamma.,.delta.-dioxovalerate transaminase from liver mitochondria. .gamma.,.delta.-Dioxovalerate transaminase catalyzes the formation of .delta.-aminolevulinic acid by a transamination reaction involving .gamma.,.delta.-dioxovalerate and alanine. To assess the significance of this alternate route, suspensions of respiring rat hepatocytes were incubated with optimal concentrations of various additives and then analyzed spectrophotometrically for synthesized porphyrins. No porphyrin synthesis was detected in cell suspensions incubated with succinate (1 mM) and glycine (20 mM). Cell suspensions incubated with .gamma.,.delta.-dioxovalerate (0.5-1.0 mM) and alanine (20 mM) synthesized 0.19 nmol of porphyrin/107 cells per 2 h (SD, 0.057). Cell suspensions incubated with .delta.-aminolevulinic acid (0.1 mM) synthesized 1.26 nmol of porphyrin/107 cells per 2 h (range, 1.18-1.32). Incubatiaons with chemically synthesized .gamma.,.delta.-dioxo[5-14C]valerate were followed by extraction and purification of porphyrin esters and heme. Liquid scintillation counting revealed radiolabel incorporation into both porphyrins and heme. These studies demonstrate significant tetrapyrrole synthesis by the .gamma.,.delta.-dioxovalerate transaminase reaction. That .gamma.,.delta.-dioxovalerate is an important precursor of heme in vivo must be considered.