Phosphofructokinase—a favourite of enzymologists and of students of metabolic regulation

Factors affecting the activation of rabbit muscle phosphofructokinase by actin

The consistent application of phosphatase inhibitors and a novel final purification step using a connected series of DE-51, DE-52, and DE-53 anion-exchange chromatography columns facilitate the preparation of electrophoretically homogeneous subpopulations of rabbit muscle phosphofructokinase which differ in their catalytic properties and endogenous covalent phosphate content. A band of "high"-phosphate enzyme (fraction II) flanked by regions of "low"-phosphate enzyme (fractions I and III) is an unusual feature of the final purification profile. Fractions I (containing in this case 0.42 mol of P/82 000 g of enzyme) and II (containing 1.26 mol of P/82 000 g of enzyme) exhibit the most pronounced functional differences of the fractions. Following our original report [Liou, R.-S., & Anderson, S. R. (1980) Biochemistry 19, 2684], both are activated by the addition of rabbit skeletal muscle F-actin. Under the assay conditions, half-maximal stimulation of phosphofructokinase activity occurs at 15.4 nM actin (in terms of monomer) for fraction I and 9.7 nM for fraction II. The low-phosphate enzyme is synergistically activated in the presence of 0.12 microM actin plus 3.0 microM fructose 2,6-bisphosphate, with a marked increase in Vmax, while the high-phosphate enzyme is not. Neither fraction is activated appreciably by the addition of G-actin or the chymotrypsin-resistant actin "core". The covalently cross-linked trimer of actin stimulates the activity of both the low- and high-phosphate enzyme fractions. However, the previously mentioned synergistic activation characteristic of fraction I fails to occur in solutions containing the trimer plus fructose 2,6-bisphosphate. Phosphorylation of fraction I in an in vitro reaction catalyzed by the cAMP-dependent protein kinase causes its properties to become more like those of fraction II. The total amount of covalent phosphate present after in vitro phosphorylation approaches 2 mol of P/82 000 g of enzyme for both fractions.

The cytochemical assay of NAD+ kinase activity in the follicular cells of guinea-pig thyroid gland

A quantitative cytochemical assay for NAD+ kinase-like activity in the guinea-pig thyroid gland is described. The NADP+ produced by the activity of the kinase was used to drive the NADP+-dependent enzyme glucose-6-phosphate dehydrogenase which is endogenous to the tissue. The activity of glucose-6-phosphate dehydrogenase is greatly in excess of that of the kinase and was unaffected by the constituents of the kinase incubation medium (ATP, Mg2+ and NAD+) either alone or in combination. Kinase activity was dependent both on ATP and Mg2+, with maximal activity seen when the Mg-ATP ratio was between 1:1 and 4:1. Free ATP inhibited the activity of the enzyme. Enzyme activity was exhibited over a broad pH range (7-9) with a peak at pH 8.2. The sulphydryl-blocking agents, p-chloromercuribenzoate, iodoacetate and iodoacetamide (at 1 mM), completely abolished kinase activity but were without effect on glucose-6-phosphate dehydrogenase activity. N-ethylmaleimide and citrate (both at 1 mM) had no effect on either kinase or glucose-6-phosphate dehydrogenase activities.