Propionate in heme biosynthesis in soybean nodules.

When soybean nodules are incubated with propionate-2-(14)C the heme moiety of leghemoglobin becomes labeled. The incorporation of propionate-2-(14)C into heme is linear with time and it appears that propionate is utilized without a lag period. The rate of incorporation of propionate-2-(14)C into heme is more rapid than the rate of incorporation of succinate-2-(14)C and citrate-1,5-(14)C, however, these rates of incorporation may be influenced by different sizes of endogenous pools of organic acids.Additional radioactive tracer experiments demonstrate that the supply of heme precursors from propionate is competitive with the supply of heme precursors from the citric acid cycle. When the concentration of propionate was high in the incubation mixture, the rate of succinate-2-(14)C incorporation into heme was decreased. Furthermore, when a large amount of succinate or acetate is added to the incubation mixture containing whole nodules, the rate of incorporation of propionate-2-(14)C into heme is reduced. The data support the hypothesis that propionate utilization makes possible a mechanism for the formation of succinyl-CoA in addition to that provided by the citric acid cycle.The fact that propionate is readily utilized by bacteroids suggested that this compound may be a normal metabolite in nodules. No detectable pool of propionate was found, however, in either soybean nodules or in isolated bacteroids suggesting that propionate, if present, is utilized as rapidly as it is formed. Experiments in which cell-free extracts of nodule bacteroids were used demonstrated the conversion of lactate to propionate. The cofactor requirements for these enzymic reactions are adenosine 5-triphosphate, Mg(++) and reduced nicotinamide adenine dinucleotide.