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P C Engel

Publications and source records attributed to P C Engel.

At least 127 records · Page 7Linked to original sources
PubMed1975

The equilibrium position of the reaction of bovine liver glutamate dehydrogenase with pyridoxal5'-phosphate. A demonstration that covalent modification with this reagent completely abolishes catalytic activity.

1. The activity of bovine liver glutamate dehydrogenase incubated with pyridoxal 5'-phosphate declined to a steady value reached within 30--60 min. The residual activity depended on the concentration of modifier up to about 5 mM. Above this concentration, however, no further inactivation was produced. The minimum activity obtainable in such incubations was 6--7% of the initial value. 2. Km values of the modified enzyme were unaltered, whereas Vmax. was decreased. 3. Activity was fully regained on dialysis against 0.1 M-potassium phosphate buffer. 4. Reduction with borohydride rendered the inactivation permanent but did not alter its extent. 5. Enzyme permanently inactivated in this way to the extent of 90% and dialysed was re-treated with pyridoxal 5'-phosphate. In this second cycle activity declined from 10 to 1% of the original activity. 6. This strongly suggests that the failure to achieve complete inactivation in a single cycle reflects a reversible equilibrium between inactive Schiff base, i.e. covalently modified enzyme, and a non-covalent complex. 7. The re-inactivation reaction occurring on dilution was demonstrated directly and a first-order rate constant obtained (0.048 min-1). This, in conjunction with an estimate of the forward rate constant for Schiff-base formation, obtained by approximate pseudo-first-order analysis of inactivation at varied modifier concentrations, gives a predicted minimum activity very close to that actually obtained in a single cycle of treatment. 8. The dissociation constant of the non-covalent complex is given by two methods as 0.90 and 1.59mM. 9. The results indicate that covalent modification with pyridoxal 5'-phosphate completely abolishes the activity of glutamate dehydrogenase.

Animals↗
PubMed1975

Modification of pig M4 lactate dehydrogenase by pyridoxal 5'-phosphate. Demonstration of an essential lysine residue.

1. Pig M4 lactate dehydrogenase treated in the dark with pyridoxal 5'-phosphate at pH8.5 and 25 degrees C loses activity gradually. The maximum inactivation was 66%, and this did not increase with concentrations of pyridoxal 5'-phosphate above 1 mM. 2. Inactivation may be reversed by dialysis or made permanent by reducing the enzyme with NaBH4. 3. Spectral evidence indicates modification of lysine residues, and 6-N-pyridoxyl-lysine is present in the hydrolsate of inactivated, reduced enzyme. 4. A second cycle of treatment with pyridoxal 5'-phosphate and NaBH4 further decreases activity. After three cycles only 9% of the original activity remains. 5. Apparent Km values for lactate and NAD+ are unaltered in the partially inactivated enzyme. 6. These results suggest that the covalently modified enzyme is inactive; failure to achieve complete inactivation in a single treatment is due to the reversibility of Schiff-base formation and to the consequent presence of active non-covalently bonded enzyme-modifier complex in the equilibrium mixture. 7. Although several lysine residues per subunit are modified, only one appears to be essential for activity: pyruvate and NAD+ together (both 5mM) completely protect against inactivation, and there is a one-to-one relationship between enzyme protection and decreased lysine modification. 8. NAD+ or NADH alone gives only partial protection. Substrates give virtually none. 9. Pig H4 lactate dehydrogenase is also inactivated by pyridoxal 5'-phosphate. 10. The possible role of the essential lysine residue is discussed.

Animals↗
PubMed1975

Ox liver glutamate dehydrogenase. The role of lysine-126 reappraised in the light of studies of inhibition and inactivation by pyridoxal 5'-phosphate.

The time-course of inactivation of bovine liver glutamate dehydrogenase by pyridoxal 5'-phosphate was studied in the presence of varied amounts of 2-oxoglutarate or NADH. Pseudo-first-order analysis reveals that the protection by both these compounds is competitive with respect to the chemical modifier. The competition is only partial, however: saturation with either NADH or 2-oxoglutarate decreases the rate constant for inactivation to a finite minimum and not to zero. Similarly, the plot of activity at equilibrium as a function of the concentration of the protecting substrate or coenzyme reveals that neither NADH nor 2-oxoglutarate protects completely against inactivation. In initial-rate experiments, pyridoxal 5'-phosphate, used as an instantaneous inhibitor rather than a long-term inactivator, displayed non-competitive inhibition with respect to both 2-oxoglutarate and NADH. These results clearly indicate that, although there is mutual hindrance between the binding to the enzyme of pyridoxal 5'-phosphate, on the one hand, and 2-oxoglutarate or NADH on the other, binding is not mutually exclusive. These findings are discussed in terms of the two-step mechanism for inactivation by pyridoxal 5'-phosphate. It is concluded that lysine-126 cannot be solely responsible for binding either the substrate or the coenzyme, but could be essential for the catalytic step.

Animals↗
PubMed1975

Horse liver alcohol dehydrogenase. A study of the essential lysine residue.

1. The inactivation of horse liver alcohol dehydrogenase by pyridoxal 5'-phosphate in phosphate buffer, pH8, at 10 degrees C was investigated. Activity declines to a minimum value determined by the pyridoxal 5'-phosphate concentration. The maximum inactivation in a single treatment is 75%. This limit appears to be set by the ratio of the first-order rate constants for interconversion of inactive covalently modified enzyme and a readily dissociable non-covalent enzyme-modifier complex. 2. Reactivation was virtually complete on 150-fold dilution: first-order analysis yielded an estimate of the rate constant (0.164min-1), which was then used in the kinetic analysis of the forward inactivation reaction. This provided estimates for the rate constant for conversion of non-covalent complex into inactive enzyme (0.465 min-1) and the dissociation constant of the non-covalent complex (2.8 mM). From the two first-order constants, the minimum attainable activity in a single cycle of treatment may be calculated as 24.5%, very close to the observed value. 3. Successive cycles of modification followed by reduction with NaBH4 each decreased activity by the same fraction, so that three cycles with 3.6 mM-pyridoxal 5'-phosphate decreased specific activity to about 1% of the original value. The absorption spectrum of the enzyme thus treated indicated incorporation of 2-3 mol of pyridoxal 5'-phosphate per mol of subunit, covalently bonded to lysine residues. 4. NAD+ and NADH protected the enzyme completely against inactivation by pyridoxal 5'-phosphate, but ethanol and acetaldehyde were without effect. 5. Pyridoxal 5'-phosphate used as an inhibitor in steady-state experiments, rather than as an inactivator, was non-competitive with respect to both NADH and acetaldehyde. 6. The partially modified enzyme (74% inactive) showed unaltered apparent Km values for NAD+ and ethanol, indicating that modified enzyme is completely inactive, and that the residual activity is due to enzyme that has not been covalently modified. 7. Activation by methylation with formaldehyde was confirmed, but this treatment does not prevent subsequent inactivation with pyridoxal 5'-phosphate. Presumably different lysine residues are involved. 8. It is likely that the essential lysine residue modified by pyridoxal 5'-phosphate is involved either in binding the coenzymes or in the catalytic step. 9. Less detailed studies of yeast alcohol dehydrogenase suggest that this enzyme also possesses an essential lysine residue.

Alcohol Oxidoreductases↗
PubMed1975

Reversible modification of pig heart mitochondrial malate dehydrogenase by pyridoxal 5'-phosphate.

1. Pig heart mitochondrial malate dehydrogenase incubated with pyridoxal 5'-phosphate at pH 8.0 and 25 degrees C gradually loses activity. Such inactivation can be largely reversed by dialysis or by addition of L-lysine or L-cysteine, and can be made permanent by NaBH4 reduction. 2. Modification of malate dehydrogenase with pyridoxal 5'-phosphate at 35 degrees C involves two phases, an initial inactivation which is reversible and a slower irreversible second stage. 3. The initial reaction between pyridoxal 5'-phosphate and malate dehydrogenase appears to involve reversible formation of a Schiff base with the epsilon-amino group of a lysine residue. 4. Inactivation of malate dehydrogenase by pyridoxal 5'-phosphate at 10 degrees C involves only the reversible reaction. 5. At 10 degrees C repeated cycles of treatment with pyridoxal 5'-phosphate and NaBH4 reduction lead to a stepwise decline in residual activity. 6. Apparent Km values for malate and NAD+ are unaltered in the partially inactivated enzyme. 7. NAD+ and NADH give only partial protection against pyridoxal 5'-phosphate inactivation. Substrates give no effect.

Borohydrides↗
PubMed1975

A product-inhibition study of bovine liver glutamate dehydrogenase.

1. Initial rates of oxidative deamination of L-glutamate with NAD+ as coenzyme, and of reductive aminiation of 2-oxoglutarate with NADH as coenzyme, catalysed by bovine liver glutamate dehydrogenase were measured in 0.111 M-sodium phosphate buffer, pH 7, at 25 degrees C, in the absence and presence of product inhibitors. All 12 possible combinations of variable substrate and product inhibitor were used. 2. Strict competition was observed between NAD+ and NADH, and between glutamate and 2-oxoglutarate. All other inhibition patterns were clearly non-competitive, except for inhibition by NH4+ with NAD+ as variable substrate. Here the extrapolation did not permit a clear distinction between competitive and non-competitive inhibition. 3. Mutually non-competitive behaviour between glutamate and NH4+ indicates that these substrates can be bound at the active site simultaneously. 4. Primary Lineweaver-Burk plots and derived secondary plots of slopes and intercepts against inhibitor concentration were linear, with one exception: with 2-oxoglutarate as variable substrate, the replot of primary intercepts against inhibitory NAD+ concentration was curved. 5. Separate Ki values were evaluated for the effect of each product inhibitor on the individual terms in the reciprocal initial-rate equations. With this information it is possible to calculate rates for any combination of substrate concentrations within the experimental range with any concentration of a single product inhibitor. 6. The inhibition patterns are consistent with neither a simple compulsory-order mechanism nor a rapid-equilibrium random-order mechanism without modification. They can, however, be reconciled with either type of mechanism by postulating appropirate abortive complexes. Of the two compulsory sequences that have been proposed, one, that in which the order of binding is NADH, NH4+, 2-oxoglutarate, requires an implausible pattern of abortive complex-formation to account for the results. 7. On the basis of a rapid-equilibrium random-order mechanism, dissociation constants can be calculated from the Ki values. Where these can be compared with independent estimates from the kinetics of the uninhibited reaction or from direct measurements of substrate binding, the agreement is reasonable good. On balance, therefore, the results provide further support for the rapid-equilibrium random-order mechanism under these conditions.

Ammonia↗
PubMed1975

Dogfish M4 lactate dehydrogenase: reversible inactivation by pyridoxal 5'-phosphate and complete protection in complexes that mimic the active ternary complex.

Dogfish M4 lactate dehydrogenase, like the corresponding pig enzyme, is inactivated by pyridoxal 5'-phosphate through modification of a single essential lysine residue. The activity is completely protected in the complexes E-NAD+-oxalate, E-NADH-oxamate and E-(NAD+-pyruvate adduct), but only partially protected in E-NAD+, E-NADH, E-NAD+-oxamate and E-NADH-oxalate.

Animals↗
PubMed1974

Protection of glutamate dehydrogenase by nicotinamide-adenine dinucleotide against reversible inactivation by pyridoxal 5'-phosphate as a sensitive indicator of conformational change induced by substrates and substrate analogues.

1. The steady residual activity of ox liver glutamate dehydrogenase at equilibrium with the reversible inactivator pyridoxal 5'-phosphate was measured in the presence and absence of various protecting agents. 2. NAD(+) (up to 15mm) and its 3-acetylpyridine analogue (up to 5mm) both failed to protect, in contrast with NADH. 3. Partial protection was given by glutarate and by succinate. Adipate and pentanoate were much less effective. 4. Correspondingly, whereas succinate and glutarate were both shown to be strong inhibitors of the catalytic reaction, competitive with glutamate, adipate was only weakly competitive, and the still weaker inhibition by pentanoate was non-competitive. 5. When the enzyme was saturated with glutarate, NAD(+) became a good, although still partial, protecting agent. In the absence of protection, 1.8mm-pyridoxal 5'-phosphate decreased enzyme activity to 9%, in the presence of 150mm-glutarate to 29%, and with glutarate and 1mm-NAD(+) only to 73%. 6. 2-Oxoglutarate also promoted protection by NAD(+), but neither pentanoate nor succinate did so. The finding with succinate is remarkable in view of findings 3 and 4 above. 7. It seems possible that substrates or analogues possessing the glutarate structure promote a conformational change that alters the mode of NAD(+) binding. This may explain why glutamate is a much better substrate than norvaline or aspartate and why negative interactions in coenzyme binding occur only in the formation of ternary complexes with glutamate or its analogues.

Animals↗
PubMed1973

The significance of abrupt transitions in Lineweaver-Burk plots with particular reference to glutamate dehydrogenase. Negative and positive co-operativity in catalytic rate constants.

1. Lineweaver-Burk plots for glutamate dehydrogenase, glucose 6-phosphate dehydrogenase and several other enzymes show one or more abrupt transitions between apparently linear sections. These transitions correspond to abrupt increases in the apparent K(m) and V(max.) with increasing concentration of the varied substrate. 2. The generalized reciprocal initial-rate equation for a multi-site enzyme requires several restrictions to be put on it in order to generate such plots. These mathematical conditions are explored. 3. It is shown that the effective omission of a term in the denominator of the reciprocal initial-rate equation represents a minimal requirement for generation of abrupt transitions. This corresponds in physical terms to negative co-operativity followed by positive co-operativity affecting the catalytic rate constant for the reaction. 4. Previous models for glutamate dehydrogenase cannot adequately account for the results. On the other hand, the model based on both negative and positive co-operativity gives a good fit to the experimental points. 5. The conclusions are discussed in relation to current knowledge of the structure and mechanism of glutamate dehydrogenase.

Biochemical Phenomena↗