Enhanced Incorporation of Tritium into Glycolate during Photosynthesis by Tobacco Leaf Tissue in the Presence of Tritiated Water

Tobacco (Nicotiana tabacum var. Havana Seed) leaf discs were allowed to photosynthesize for 3 to 20 minutes in the presence of (14)CO(2) and (3)H(2)O. Several metabolites of the Calvin cycle and photorespiratory pathway were isolated and purified and the (3)H:(14)C values measured. Glycolate had a 5- to 10-fold higher (3)H:(14)C than the Calvin cycle intermediate 3-phosphoglyceric acid, or its end product sucrose. The glycolate oxidase inhibitor alpha-hydroxy-2-pyridinemethanesulfonic acid caused glycolate to accumulate in the tissue and lowered the (3)H:(14)C in glycolate to a value similar to that in 3-phosphoglyceric acid. Phosphoglycolate, a possible precursor of glycolate arising from the Calvin cycle, exhibited a (3)H:(14)C value similar to 3-phosphoglyceric acid under all conditions. The finding of a (3)H enrichment in glycolate suggests that another source of glycolate, possibly the reduction of glyoxylate, exists in leaf tissue. Analyses of incorporation of (3)H into the pro-2R and pro-2S hydrogens of glycolate, in the presence and absence of alpha-hydroxy-2-pyridinemethanesulfonic acid, suggest an alternative source of glycolate. Biochemical mechanisms to account for (3)H enrichment into glycolate are evaluated.