The consequences of the isotope effect on proline dehydrogenation rates estimated by the tritium loss method

Loss of tritium from a substrate is often used to estimate the rate of dehydrogenation. However, loss of 3H may be much slower than loss of H because of the tritium isotope effect. In order to assess the impact of the tritium isotope effect, loss of 3H from the C-5 position of proline during dehydrogenation by rat liver mitochondria and bacteroids from soybean (Glycine max [L.] Merrill) nodules was compared with appearance of 14C in products of [14C]proline dehydrogenation. Incubations were carried out in the presence of o-aminobenzaldehyde (added to trap the initial product, delta 1-pyroline-5-carboxylate). The fraction of total 14C products trapped by o-aminobenzaldehyde varied from 0.07 to 0.75 depending upon experimental conditions. With rat liver mitochondria, dehydrogenation of [14C]proline was between 3.27 and 9.25 times faster than dehydrogenation of 3H proline, depending upon assay conditions. Soybean nodule bacteriods dehydrogenated [14C]proline about 5 times faster than [3H]proline. We conclude the following: (i) the rate of proline dehydrogenation may be greatly underestimated by the tritium assay because of the tritium isotope effect, and (ii) the 14C assay may underestimate the rate of proline dehydrogenation if it is assumed that o-aminobenzaldehyde quantitatively traps delta 1-pyrroline-5-carboxylate under all conditions. The simplicity of the tritium assay makes it attractive for routine use. However, its use requires determination of the tritium isotope effect, under the specific conditions of the assay, in order to correct the results. The considerations discussed here have broad applicability to any dehydrogenase assay employing tritium loss.