“…Since the size of the steady-state NH~" pool depends on the ratio of NH~ influx over NH 3 efflux plus metabolism, an additional NH~ import should increase the NH2 pool. Alternatively, even in the absence of a second carrier, this ratio might be higher for NH~ than for MA, if the natural substrate (NH~) is transported faster than the analog (MA), as shown for some strains [9,11,50]. As will be discussed in section 6.3., the back diffusion rate of both molecules is about equal.…”