REGULATION OF ALTERNATIVE OXIDASE ACTIVITY BY PYRUVATE IN SOYBEAN MITOCHONDRIA

The regulation of alternative oxidase activity by the effector pyruvat e was investigated in soybean (Glycine max L.) mitochondria using deve lopmental changes in roots and cotyledons to vary the respiratory capa city of the mitochondria. Rates of cyanide-insensitive oxygen uptake b y soybean root mitochondria declined with seedling age. Immunologicall y detectable protein levels increased slightly with age, and mitochond ria from younger, more active roots had less of the protein in the red uced form. Addition of pyruvate stimulated cyanide-insensitive respira tion in root mitochondria, up to the same rate, regardless of seedling age. This stimulation was reversed rapidly upon removal of pyruvate, either by pelleting mitochondria (with succinate as substrate) or by a dding lactate dehydrogenase with NADH as substrate. In mitochondria fr om cotyledons of the same seedlings, cyanide-insensitive NADH oxidatio n was less dependent on added pyruvate, partly due to intramitochondri al generation of pyruvate from endogenous substrates. Cyanide-insensit ive oxygen uptake with succinate as substrate was greater than that wi th NADH, in both root and cotyledon mitochondria, but this difference became much less when an increase in external pH was used to inhibit i ntramitochondrial pyruvate production via malic enzyme. Malic enzyme a ctivity in root mitochondria declined with seedling age. The results i ndicate that the activity of the alternative oxidase in soybean mitoch ondria is very dependent on the presence of pyruvate: differences in t he generation of intramitochondrial pyruvate can explain differences i n alternative oxidase activity between tissues and substrates, and som e of the changes that occur during seedling development.