Role of Hexose Monophosphate Pathway in Tomato Catabolism

Abstract

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 Materials & MethodsThe tomato fruit used in the present experiments were of the Michigan State Forcing variety and were selected on the same basis as described in earlier work (3, 9).1 Carbon-14 Labeled Substrates: Gluconate-1-C14 was purchased from the Nuclear-Chicago Corp. Gluconate-2, and -6-C14 were prepared from the correspondingly labeled glucose samples by the method of

Monitoring flux through the oxidative pentose phosphate pathway using [1- 14 C]gluconate

“…Detailed analysis of the pattern of labelling within CO 2 and ethanol under anaerobic conditions was consistent with gluconate being converted to 6-phosphogluconate and thereafter metabolised via the OPPP. Subsequent studies on ripening tomato fruit (Wang et al 1962) and mung bean hypocotyls (Ashihara and Komamine 1975) reached similar conclusions. In the proposed scheme of metabolism, C1 of gluconate is released uniquely during the decarboxylation of 6-phosphogluconate to ribulose 5-phosphate by 6-phosphogluconate dehydrogenase, and thus should provide a direct measure of flux through the irreversible oxidative steps of the pathway.…”

“…Of course, application of this technique depends on the ability of the tissue to absorb and metabolise exogenous gluconate. However, in addition to arabidopsis suspension cultures (this paper), maize coleoptile (Beevers 1956), tomato fruit (Wang et al 1962) and mung bean hypocotyls (Ashihara and Komamine 1975), we have observed that maize primary root tips, potato tubers, tobacco leaves and mango mesocarp release appreciable amounts of 14 CO 2 when incubated with [1-14 C]gluconate (unpublished data). This suggests that the capacity to absorb and metabolise gluconate is widespread in higher-plant tissues and that the technique described here is broadly applicable.…”

Presence and activity of the hexosemonophosphate shunt in a marine pennate chemoorganotrophic diatom, Nitzschia alba, clone Link 001

“…Katz and Wood (1963) consider fructose-6-phosphate in non-growing tissue as completely catabolized via the hexose-6-phosphate pool associated with the EMP. Whereas Wang et al (1962), while considering it a component of the hexose-6-phosphate pool of the EMP in growing tissue, conclude that it is not a complete equilibrium with it and therefore passes through both the HMP and the EMP in a ratio established by the starting glucose-glucose concentrations. This model then considers that the HMP pathway is contributing intermediates for biosynthesis required for growth and division (Wanget a/.…”

“…Monophosphnte Shunt crnct the EmbdeilMeyerhgf-Parnas Pnth~vnys Radiorespirometric evaluation of the HMP and the EMP pathways by I4CO, trapping from [I4Cl]-or [ t 4 C 6 ] -g l~~o s e or [14C6]gluconate can be interpreted using the methods of either Katz and Wood (1963) or Wang et a / . (1962).…”

Host Physiology and Obligate Fungal Parasites1)

“…Using a range of tissues, they showed that utilization of the pentose phosphate sequence proceeded parallel with biosynthesis of nucleic acids. In tomato fruit the pentose phosphate sequence is considered to serve mainly as a mechanism for converting glucose to intermediates for biosynthesis (WANG, DOYLE, and RAMSEY 1962).…”