Glutamine and Glutamate as Respiratory Substrates of Rabbit Reticulocytes

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Abstract
In rabbit reticulocytes there is formed 4–16 times as much 14CO2 from tracer amounts of [U‐14C]glutamine than from [U‐14C]glutamate. 14CO2 formation from substrate amounts of [U‐14C]glutamine and [U‐14C]glutamate is concentration dependent according to a Michaelis‐Menten relation. The apparent Km is 2.0 mM for glutamine and 40 mM for glutamate. The maximal value of the relative 14CO2 formation (14CO2 formed as a percentage of O2 consumed) reaches 80% for both glutamine and glutamate. Under physiological conditions, i.e. at its average plasma concentration, glutamine furnishes 15–50% of the respiratory CO2 formed by the reticulocyte. It is the most important exogenous substrate. The kinetic data are consistent with the assumption that the CO2 formation from glutamine proceeds via glutamate. The large quantitative difference between glutamine and glutamate may be explained by the vastly greater permeation of glutamine as compared with that of glutamate.