New reaction sequences for the non-oxidative pentose phosphate pathway

Abstract

1. Reactions leading to the formation of 14C-labelled volatile compounds and compounds volatile under acid conditions were investigated in a system actively synthesizing hexose 6-phosphates from [U-14C]ribose 5-phosphate by reactions catalysed by enzymes prepared from acetone-dried powder of rat liver; no reactions involving 14C-labelled volatile compounds were detected. Similarly the fixation of 14C-labelled volatile compounds into hexose 6-phosphate could not be detected. 2. A complete carbon balance was made for the reactants, intermediates and products of the reactions involved in the conversion of ribose 5-phosphate into hexose 6-phosphate by enzymes of rat liver. Five additional intermediates of pentose 5-phosphate metabolism in liver were detected, namely D-manno-heptulose 7-phosphate, D-altro-heptulose 1,7-bisphosphate, D-glycero-D-ido-octulose 1,8-bisphosphate, D-glycero-D-altro-octulose 1,8-bisphosphate and D-arabinose 5-phosphate. 3. D-Arabinose 5-phosphate was found to be utilized by a rat liver enzyme preparation to produce both hexose 6-phosphate and triose phosphate. 4. D-Arabinose 5-phosphate was reversibly converted into other pentose 5-phosphates. Paper chromatographic and enzymic evidence indicated that the conversion involved an enzyme tentatively named arabinose phosphate 2-epimerase, which catalyses the following reaction: D-arabinose 5-P in equilibrium D-ribose-5-P. 5. A variety of rat tissues also utilized D-arabinose 5-phosphate to produce both hexose 6-phosphate and triose phosphate and at a rate comparable with that obtained with D-ribose 5-phosphate. 6. A new reaction sequence for the non-oxidative pentose phosphate pathway in liver is proposed.

Regulation of hepatic altro heptulose 1,7‐bisphosphate levels and control of flux through the pentose pathway by fructose 2,6‐bisphosphate

“…Its concentration in control liver perfusions was ~10 nmol/g tissue ( fig.2) which is similar to 5-7 nmol/g tissue found in [20]. This intermediate plays a r61e in the proposed L-type pentose pathway [35,36], and the existence of octulose phosphates [19,20] support the operation of this pathway. However, there does appear to be some doubt regarding the metabolic fate of arabinose 5-P another key intermediate in the L-type pentose pathway [33,34].…”

“…In the glucagon treated liver, the increased altro heptulose 7-P concentration would be due to the same set of reactions as above, together with a decreased utilization ofaltro heptulose 7-P by phosAlthough there appears to be some debate regarding the mechanism of the non-oxidative pentose phosphate pathway in liver [31][32][33][34], there is very clear evidence for the existence in liver tissue of altro heptulose 1,7-P2 (here and [19,20]). Its concentration in control liver perfusions was ~10 nmol/g tissue ( fig.2) which is similar to 5-7 nmol/g tissue found in [20].…”

“…altro Heptulose 1,7-P2 is not considered to be an intermediate of the pentose phosphate pathway as seen in current text books, however recent studies have shown that it is present in liver tissue and can be formed by liver enzymes in vitro [19][20][21]. altro Heptulose 1,7-P2 does play a key role in the path of carbon in photosynthesis [22].…”

Properties and functions of the thiamin diphosphate dependent enzyme transketolase

“…4) [183]. Based mainly on carbon balance studies and on experiments involving tracing of 14 C labelled sugars, Williams and coworkers have proposed an alternative version of the PPP, the so called`L -type'', which apparently operates in hepatocytes from rats [178]. In this type of the PPP glucose 6-phosphate also acts as an acceptor substrate for TK, resulting in the generation of an octulose [66,179,180].…”

Pentose pathway in human liver.

“…Thus, with unlabeled ribulose 5-P being formed in the oxidative portion from unlabeled glucose, here from the glucose load, and [1-14C]ribose, isotopic nonequilibration of the pentose 5-phosphates, resulting in higher specific activities of ribose 5-P than of xylulose 5-P, explains the incorporations into carbon 3 related to those in carbon 1 (17). Williams et al (21) reported that 14C was incorporated into carbon 2 ofglucose 6-P when (1-14C]ribose 5-P was incubated for short periods with a rat liver enzyme preparation. Reactions yielding such incorporation, with the oxidative portion ofthe pentose pathway, comprise the L-type pathway that he and his coworkers proposed ( Fig.…”