Photosynthesis in the Higher Plant, Vicia faba: III. Serine, a Precursor of the Tricarboxylic Acid Cycle.

Evidence is presented to support the hypothesis that serine, rather than 3-phosphoglycerate of the Calvin cycle, is a precursor of the tricarboxylic acid cycle during photosynthesis by the higher plant, Vicia faba. Identification of the serine intermediate is based upon a unique C(1) > C(2) > C(3) isotope distribution for that metabolite following the fixation of (14)CO(2). This labeling pattern, while incompatible with an origin either in the Calvin cycle or the glycolate pathway, satisfies a critical criterion for the 3-carbon precursor of the anomalously labeled organic acids. The predominant carboxyl carbon atom labeling of serine reflects either a mixing of two pools of that metabolite, ie., C(1) = C(2) > C(3) and C(1) > C(2) = C(3), or a higher order of complexity in its synthesis. An anomalous C(1) = C(2) > C(3) < C(4) distribution for aspartate, however, suggests an origin by the carboxylation of a 3-carbon intermediate related to serine which has a C(1) = C(2) > C(3) distribution. The latter distribution has been proposed for the serine intermediate of the postulated formate pathway. This pathway is described by the generalized metabolic sequence: CO(2) --> formate --> serine --> organic acids. Corresponding carbon atom distributions for citrate (C(1) > C(2)), aspartate (C(2) > C(3)), and serine (C(2) > C(3)) belie a precursor-product relationship with alanine (C(2) = C(3)), which is a molecular parameter of the Calvin cycle product, 3-phosphoglycerate.