Abstract
Uptake of [(14)C]putrescine, [(14)C]arginine, and [(14)C]ornithine was measured in five separate callus cell lines of Zea mays. Each precursor was rapidly taken into the intracellular pool in each culture where, on the average, 25 to 50% of the total putrescine was found in a conjugated form, detected after acid hydrolysis. Half-maximal labeling of each culture was achieved in less than 1 minute. Within this time frame of precursor incorporation, only putrescine derived from arginine was conjugated, indicating that putrescine pools derived from arginine may initially be sequestered from ornithine-derived putrescine. The decarboxylase activities were measured in each culture after addition of exogenous polyamine to the growth medium to assess differential regulation of the decarboxylases. Arginine and ornithine decarboxylase activities were augmented by added polyamine, the effect on arginine decarboxylase being eightfold greater than on ornithine decarboxylase. Levels of extractable ornithine decarboxylase were consistently 15- to 100-fold higher than arginine decarboxylase, depending on the titer of extracellular polyamine. Taken as whole the results support the idea that there are distinct populations of polyamine that are initially sequestered after the decarboxylase reactions and that give rise to separate end products and possibly have separate functions.