Role of Hexose Monophosphate Pathway in Tomato Catabolism

The operation of the hexose monophosphate pathway (HMP) in plants has been the object of extensive studies in recent years (1, 2, 4). In previous papers (3, 9) it has been demonstrated that in intact tomato fruit, glucose is catabolized mainly by way of the Embden-Meyerhof-Parnas pathway (EMP) and to a limited extent via the HMP pathway. However, the catabolic fate of the pentose phosphate, derived from glucose via the HMP pathway, remains to be elucidated. The latter problem is an important one since one of the assumptions made in deriving equations for pathway estimation is that the pentose phosphate does not participate significantly in fruit respiration (3). Subsequently, from radiorespirometric studies of gluconate catabolism in tomato fruit (9) it is revealed that C-2, C-3,4, and C-6 of gluconate, in addition to C-1. were converted to some extent to respiratory CO, a fact indicating that the pentose phosphate derived from glucose via phosphogluconate was, indeed, engaged in respiratory functions. In the present work, the incorporation of gluconate-2-C14, gluconate-6-C14, and ribose-1-C14 into fruit constituents, particularly that into the fruit glucose molecules, has been studied. The isotopic distribution patterns in fruit glucose derived from the respectively labeled substrates, enables one to gain considerable insight into the catabolic mechanism responsible for the conversion of the carbon atoms of pentose phosphate to respiratory CO.,. The equations devised previously for the estimation of pathway participations (3) are revised taking into consideration the findings in the present study.