The effect of calmodulin and far-red light on the kinetic properties of the mitochondrial and microsomal calcium-ion transport system from corn.

The kinetic properties of active Ca(2+) transport into mitochondria and microsomal membrane vesicles prepared from coleoptiles of dark-and light-grown corn seedlings have been studied. The apparent values for K m and V max for Ca(2+) of the mitochondrial transport system from dark-grown plants are about one order of magnitude higher than those from the microsomal transport system. Calmodulin has no effect on the Ca(2+) accumulation into mitochondria whereas the apparent maximum transport velocity and affinity for Ca(2+) of the microsomal Ca(2+)-transport system are both increased by calmodulin. When intact corn seedlings are irradiated with far-red light, the calmodulin-induced increase of the apparent maximum transport velocity and affinity for Ca(2+) can no longer be observed. From these data it can be concluded that the low cytoplasmic Ca(2+) concentration in the cytoplasm of coleoptile cells from dark-grown corn is maintained by a calmodulin-regulated Ca(2+) pump. Irradiation with photomorphogenically active far-red light lowers the Ca(2+)-transport activity and thus causes an increase of the cytoplasmic, free-Ca(2+) concentration. The physiological implications will be discussed.