Carrier-mediated Uptake of Arginine and Urea by Volvox carteri f. nagariensis

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Abstract
Volvox carteri f. nagariensis takes up arginine via a high affinity, highly specific carrier, whereas carriers for neutral and acidic amino acids cannot be detected (even in nitrogen-starved cultures). Exogenous arginine is accumulated against a steep concentration gradient and is incorporated into protein with high efficiency, but it is not catabolized to any significant extent and will not serve as a nitrogen source adequate to support growth. Urea is also taken up by a saturable carrier, but several lines of evidence indicate that the arginine and urea carriers are distinct and different. Preexposure to arginine suppresses arginine uptake while stimulating urea uptake. The Ki values observed for reciprocal, competitive inhibition of uptake by arginine and urea are orders of magnitude different from the respective Km values for uptake. The two uptake systems show entirely different patterns of sensitivity to inhibition by structural analogs. Finally, the Vmax values for arginine and urea uptake fluctuate independently (but in a regular pattern) during the asexual life cycle. The fluctuations of urea uptake activity are of considerable magnitude and appear to be linked to key phases of the developmental program.

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