An investigation of the mechanism and quantitative contribution of the pentose phosphate pathway in the
glucose metabolism of
Morris Hepatoma 5123C is reported.
Morris Hepatoma 5123C has an active non-oxidative segment of
pentose pathway as judged by its ability to convert
ribose 5-P to
hexose 6-P in a standard assay. Based on compliance with qualitative and quantitative criteria, the cells exhibit the L-type
pentose pathway reaction sequence rather than the F-type pathway. This compliance included the formation of intermediates characteristic of the L-type pathway, namely
arabinose 5-P, octulose mono- and bisphosphates and
sedoheptulose 1,7-bisphosphate, during the dissimilation of
ribose 5-P to
hexose 6-P. The intermediary role of
arabinose 5-P was suggested by the incorporation of its
carbon into various intermediates and products of the
pentose pathway. Intermediary roles for ido octulose mono- and bisphosphates were supported by their participation in the reaction catalyzed by the
phosphotransferase enzyme of the L-type
pentose pathway. Presence of L-type PP reactions was further affirmed by 14C-prediction labelling experiments using [5-14C]- and [2-14C]
glucose as specifically labelled substrates. Using two methods of measurement, the F-type
pentose cycle made a negligibly small contribution to
glucose metabolism, while the measured value of the L-type
pentose pathway accounted for 30% (approx.) of the total
glucose metabolism of these cells, a value consistent with the high activity of the
enzymes of the L-type
pentose pathway in
Morris Hepatoma 5123C cells and the very high activity of the non-oxidative segment of the pathway in vitro. The findings validate the proposal that the L-type
pentose pathway reactions constitute the non-oxidative segment of the pathway in
Morris Hepatoma 5123C cells. Reasons involving
pyruvate recycling reactions show why there is low incorporation of 14C-isotope in C-1 of
glucose 6-P, when [4,5,6-14C]
glucose and [6-14C]
glucose are L-type PP test substrates in intact cells.