The metabolism of acetol phosphate. I. Synthesis, properties and enzymic reduction

Net glucose production from acetone in isolated murine hepatocytes. The effect of different pretreatments of mice

1. To evaluate the condition under which net glucose production from acetone, added as sole substrate, occurs different pretreatments of mice, in combination with starvation, were used; (i) acetone pretreatment (acetone is a known inducer of cytochrome P-450 isozymes involved in this pathway), (ii) fructose pretreatment (to induce NADPH+H+ generating enzymes) or (iii) their combination. 2. There was net glucose formation from acetone only in that case, when the cells were prepared from 48 hr fasted animals pretreated with both acetone and fructose. However, using 2-14C-acetone, incorporation of 14C-carbon into glucose could be detected in all the cases and, at the same time, acetone was without any effect on protein synthesis. 3. The addition of acetone increased gluconeogenesis from alanine in almost all the cases. The only exception from this general rule was that the case, when hepatocytes were prepared from acetone pretreated 48 hr starved mice where, instead of the elevation of glucose formation, a decrease of that was caused by acetone. 4. Acetone decreased 14C-carbon incorporation into glucose from 14C-(U)-alanine added at saturating concentration in hepatocytes prepared from starved mice. 5. Similarly to acetone there was no net glucose formation from acetone either when added alone, however, it enhanced gluconeogenesis from alanine at non-saturating concentrations of the amino acid. 6. Methylglyoxal proved gluconeogenic in all the cases. 7. It is concluded that net glucose formation from acetone as sole substrate occurs only under those conditions which are far from a physiological situation, however, when gluconeogenesis from another substrate takes place, acetone can contribute to net glucose formation in hepatocytes prepared from fasted mice.

Specificity and kinetics of triose phosphate isomerase from chicken muscle

The isolation of crystalline triose phosphate isomerase from chicken breast muscle is described. The values of k(cat.) and K(m) for the reaction in each direction were determined from experiments over wide substrate-concentration ranges, and the reactions were shown to obey simple Michaelis-Menten kinetics. With d-glyceraldehyde 3-phosphate as substrate, k(cat.) is 2.56x10(5)min(-1) and K(m) is 0.47mm; with dihydroxyacetone phosphate as substrate, k(cat.) is 2.59x10(4)min(-1) and K(m) is 0.97mm. The enzyme-catalysed exchange of the methyl hydrogen atoms of the ;virtual substrate' monohydroxyacetone phosphate with solvent (2)H(2)O or (3)H(2)O was shown. This exchange is about 10(4)-fold slower than the corresponding exchange of the C-3 hydrogen of dihydroxyacetone phosphate. The other deoxy substrate, 3-hydroxypropionaldehyde phosphate, was synthesized, but is too unstable in aqueous solution for analogous proton-exchange reactions to be studied.

Dihydroxyacetone phosphate. Its structure and reactivity with -glycerophosphate dehydrogenase, aldolase and triose phosphate isomerase and some possible metabolic implications

1. Dihydroxyacetone phosphate exists in neutral aqueous solution at 20 degrees C as a mixture of keto, gem-diol and enolic forms in the ratio 55:44:1. 2. The three forms are freely interconvertible and rate constants for these reactions have been determined. 3. Keto-dihydroxyacetone phosphate is the primary reactive species in the reactions catalysed by alpha-glycerophosphate dehydrogenase, aldolase and triose phosphate isomerase. 4. The proportion of keto form to gem-diol forms of dihydroxyacetone phosphate is temperature-dependent. At 37 degrees C, 83% is keto-dihydroxyacetone phosphate. 5. The enzymological and metabolic consequences of these results are discussed.