Determination of enzyme kinetic parameters by continuous addition of substrate to a single reaction mixture and analysis by a tangent-slope procedure. I. Analysis of the method using computed progress curves

Enzymes of carnitine acylation. Is overt carnitine palmitoyltransferase of liver peroxisomal carnitine octanoyltransferase?

Liver mitochondria prepared by differential centrifugation are contaminated by significant quantities of peroxisomes and microsomal fractions. 'Easily solubilized carnitine palmitoyltransferase' prepared from liver mitochondria is thought to originate from the outer surface of the mitochondrial inner membrane. We have characterized the carnitine palmitoyltransferase activities of freeze-thaw extracts of rat liver mitochondrial preparations. Chromatography on Sephadex G-100 yields two broad peaks of carnitine decanoyltransferase activity: one eluted at the end of the void volume, which can be removed (precipitated) by ultracentrifugation; the second peak represents the soluble activity and is eluted at an Mr near 70,000. The activity in the soluble peak is precipitated by an antibody raised against carnitine octanoyltransferase purified from mouse liver peroxisomes. In contrast, antibody raised against carnitine palmitoyltransferase purified from liver mitochondrial membranes had no effect (P. Brady & L. Brady, personal communication). The carnitine acyltransferase activities of the Mr-70,000 peak in the presence or absence of Tween 20 showed maximum activity with decanoyl-CoA and about one-third of this activity with palmitoyl-CoA, similar to peroxisomal carnitine octanoyltransferase. These data show that 7500 g preparations of liver mitochondria isolated by differential centrifugation are enriched by peroxisomal carnitine octanoyltransferase (approx. 20% of the protein of the fraction is peroxisomal) and indicate that this enzyme may be the one reported as 'overt' or 'easily solubilized' mitochondrial carnitine palmitoyltransferase.

Activation and oligomerization of immobilized biodegradative L-threonine dehydrase

Biodegradative L-threonine dehydrase of Escherichia coli (radio labeled with [3H]pyridoxine) was immobilized on CNBr-activated Cl-Sepharose. The specific catalytic activity and S0.5 values of the matrix-bound dehydrase in the presence of AMP were similar to those of the soluble oligomeric enzyme in the presence of AMP (matrix-bound, associated dehydrase). When the bound dehydrase was washed with AMP-free buffer, about 50% of the bound dehydrase was removed and about 50% remained attached, as measured by radioactivity. The resulting matrix-bound, dissociated dehydrase possessed activity in the absence of AMP as is characteristic of soluble, unactivated dehydrase. The bound, dissociated dehydrase was capable of binding nearly an equal amount of soluble dehydrase in the presence of AMP; this treatment raised the specific enzyme activity of the bound dehydrase to 83% of that of the original matrix-bound, associated dehydrase. These observations correlate with the effects of AMP on the activity and quaternary structure of soluble dehydrase. When AMP was added to the matrix-bound, dissociated dehydrase, the activation observed was only a small fraction of that obtained with soluble dehydrase plus AMP. The failure of AMP to activate the major fraction of immobilized dehydrase monomer strongly suggests that dimerization is required in the activation by AMP.

The identity of hexokinase activities from mitochondrial and cytoplasmic fractions of rat brain homogenates

Cytoplasmic hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) was purified from the soluble fraction of a rat brain homogenate by a procedure that included a unique affinity elution of the enzyme from Blue Dextran-Sepharose. The purified enzyme was examined with respect to properties in which the impure cytoplasmic enzyme has been reported to differ from the solubilized mitochondrial enzyme. These included the ability to bind to mitochondria, inhibition by quercetin, effect of pH on activity, and kinetics. In all regards the purified mitochondrial and cytoplasmic enzymes appeared identical. In addition, comparative peptide maps after partial proteolysis showed no detectable differences. These results do not support the view that there exist distinct mitochondrial and cytoplasmic forms of hexokinase, the latter being permanently relegated to a cytoplasmic location and unable to participate in a dynamic equilibrium with the mitochondrially-bound enzyme. Alternatives are proposed to explain previous results that had been interpreted as indirect evidence for the existence of a distinct cytoplasmic hexokinase.