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Mechanisms for Urea Uptake by the Diatom Phaeodactylum tricornutum: The Uptake of Thiourea
T. A. V. Rees and P. J. Syrett
The New Phytologist
Vol. 83, No. 1 (Jul., 1979), pp. 37-48
Stable URL: http://www.jstor.org/stable/2433743
Page Count: 12
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Thiourea inhibited competitively the uptake of urea by urea-grown cells of Phaeodactylum with a Ki of 180 μ M. Urea-grown cells also absorbed and accumulated thiourea. Uptake of thiourea resembled that of urea in that (i) uptake by ammonium-grown cells was slow, (ii) uptake was much increased by a period of nitrogen starvation, (iii) uptake was stimulated by light and (iv) at low substrate concentrations uptake was inhibited almost completely by 0.1 mM CCCP (carbonyl cyanide m-chlorophenylhydrazone). It is concluded that thiourea and urea are taken up by the same active mechanism. With nitrogen-depleted cells, the relationship between rate of thiourea uptake and thiourea concentration was hyperbolic with a half-saturation constant of 170 ± 33 μ M and a Vmax of 580 nmol thiourea 108 cells-1 h-1. This half-saturation constant is about 200 times greater than the value for urea uptake by nitrogen-depleted cells although the Vmax value is about the same. The relationship between passive and active uptake is discussed. Because of the low affinity of the active uptake mechanism for thiourea, passive uptake is quantitatively important at low thiourea concentrations; this is particularly so with nitrogen-replete cells in which the capacity of the active mechanism is less well developed. As much of the urea taken up by Phaeodactylum remains in the cells as free urea, it is preferable to study the urea uptake mechanism by measuring the uptake of urea directly. In contrast, organisms like Chlorella metabolize [14C]urea rapidly and conclusions may be ambiguous if measurement of 14C within the cells is used as a measure of uptake. With Chlorella, it may be preferable to use the analogue, [14C]thiourea, to study the uptake mechanism and, in this organism, the difference in affinities of the active uptake mechanism for urea and thiourea is not so great as it is in Phaeodactylum.
The New Phytologist © 1979 New Phytologist Trust