Comparison of bone and osteosarcoma adenylate cyclase. Partial purification of membranes and kinetic properties of enzyme

A model for the regulation of brain adenylate cyclase by ionic equilibria

Multiple-equilibrium equations were solved to investigate the individual and separate effects of Mg2+, Mn2+, Ca2+, ATP4-, and their complexes on the kinetics of brain adenylate cyclase. The effects of divalent metals and/or ATP4- (in excess of their participation in complex formation) were determined and, from the corresponding apparent affinity values, the following kinetic constants were obtained: Km(MgATP) = 1.0 mM, Ki(ATP4-) = 0.27 mM, Km(MnATP) = 0.07 mM, and Ki(CaATP) = 0.015 mM. MgATP, MnATP, ATP4-, and CaATP were shown to compete for the active site of the enzyme. Hence, it is proposed that endogenous metabolites with a strong ligand activity for divalent metals, such as citrate and some amino acids, become integrated into a metabolite feedback control of the enzyme through the release of ATP4- from MgATP. Ca2+ fluxes may participate in the endogenous regulation of adenylate cyclase by modifying the level of CaATP. The free divalent metals show an order of affinity K0.5(Ca2+) = 0.02 mM, K0.5(Mn2+) = 3.8 mM, K0.5(Mg2+) - 4.7 mM, and an order of activity Mn2+ greater than Mg2+ greater than Ca2+. The data indicate that Mn2+ and Mg2+ ions may compete for a regulatory site distinct from the active site and increase Vm without changing Km(MgATP), Km(MnATP), or Ki(ATP4-). The interactions of ATP4- and CaATP, which act as competitive inhibitors of the reaction of the enzyme with the substrates MgATP and MnATP, and Mg2+ and Mn2+, which act as activators of the enzyme in the absence of hormones, are shown to follow the random rapid equilibrium BiBi group-transfer mechanism of Cleland with the stipulation that neither Mg2+ nor Mn2+, in excess of their respective participation in substrate formation, are obligatorily required for basal activity. ATP4- and CaATP are involved in dead-end inhibition. For MgCl2 saturation curves at constant total ATP concentration, the computer-generated curves based on the RARE BiBi model predict a change in the Hill cooperativity h from a basal value of 2.6, when Mg2+ is not obligatorily required, to 4.0 when the addition of hormones or neurotransmitters induces an obligatory requirement for Mg2+.

Formycin 5'-triphosphate, a fluorescent analog of ATP, as a substrate for adenylate cyclase

Formycin 5'-triphosphate (FoTP), a fluorescent analog of ATP, is shown to be a substrate for the membrane-bound adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] from rat osteosarcoma cells. The formation of the adenylate cyclase reaction product, 3',5'-cyclic formycin monophosphate (cFoMP), was followed by the conventional radioimmunoassay (RIA) procedure used to detect cAMP and by an assay procedure in which the reaction product was separated from the substrate by reverse-phase high-pressure liquid chromatography (HPLC) and the reaction product was detected by fluorometry. Because the HPLC--fluorometric procedure can determine the amount of cFoMP present in the reaction mixture within 6 min, the enzymatic conversion of FoTP to cFoMP can be followed directly during the course of a typical 15-min incubation. The amount of cFoMP detected by this procedure was found to be within 2% of the values obtained by the RIA. The rate of product formation with FoTP was similar to that observed with ATP and the activity of the enzyme was enhanced about 5-fold with guanyl-5'-yl imidodiphosphate when either ATP or FoTP was used as the substrate. Kinetic studies revealed values for the Vmax of 120 pmol/min per mg of protein and apparent Km values of 220 microM with both substrates. In addition to suggesting that the recognition of the substrate by the adenylate cyclase may not require a specific chemical structure of the 5-membered ring of the base or a unique configuration about either the glycosyl or the C(5')-C(4') bond, the results of this study are consistent with the idea that the cytotoxicity observed with the adenosine analog formycin may be the result of its metabolism to cFoMP. Furthermore, these studies indicate that the fluorescent analog FoTP can be used, in combination with HPLC, to provide an alternative, nonradioactive direct method for the assay of adenylate cyclase catalytic activity.

Osteosarcoma cytosol factor promotes parathyroid hormone stimulation of adenylate cyclase independent of GTP

The adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1)-stimulating factor from rat osteosarcoma cytosol was purified 600-fold by ion-exchange chromatography. The factor has an apparent Mr of 20000, is cold-labile, but retains activity at -20 degrees C in 10% glycerol. The factor enhanced parathyroid hormone stimulation of adenylate cyclase and restored hormone responsiveness to membranes washed with 0.5 M NaCl. These 'GTP-like' effects were not inhibited by 100 microM GDP-beta-S, which completely abolished the GTP enhancement of both basal and hormone-stimulated adenylate cyclase. Adenylate cyclase activity in the presence of the stimulating factor was linear with time, and showed hyperbolic dependence on factor concentration. The factor also linearized (in double reciprocal plots) the downward-concave Mg2+-dependence of adenylate cyclase, increasing the apparent affinity of the enzyme for Mg2+. The presence of the factor in two clonal osteosarcoma cell lines correlated with parathyroid hormone-stimulatable adenylate cyclase. Factor stimulation was absent while GTP stimulation was retained in the hormone-nonresponsive clone. Factor and hormone sensitivity were restored by in vivo passage. This factor thus may represent a guanyl nucleotide-independent path for cellular regulation of hormone response.

Comparison of bone and osteosarcoma adenylate cyclase. Effects of Mg2+, Ca2+, ATP4- and HATP3- in the assay mixture

The effects of Mg(2+) and Ca(2+) on bone and osteosarcoma adenylate cyclase were investigated. The concentrations of the cations and other ionic species in the assay mixture were calculated by solving the simultaneous equations describing the relevant ionic interactions (multiple equilibria). We re-examined the effects of HATP(3-) and ATP(4-) on enzyme activity and found that (i) the concentration of the minor ATP species is less than 1% of that of MgATP(2-), and their ratio to MgATP(2-) is constant if Mg(2+) and H(+) concentrations are unchanged; (ii) Mg(2+) addition decreased the ratio of the minor species to MgATP(2-) and increased the enzyme activity, but no meaningful kinetic model could attribute this effect of HATP(3-) or ATP(4-). On the other hand, kinetic analysis of Mg(2+) effects showed: (i) stimulation via two metal sites, separate from the catalytic (MgATP(2-)) site, with apparent K(m) values of approximately 1 and 8mm; (ii) that the low affinity increased towards the higher one when the enzyme activity rose as a result of increased substrate or guanine nucleotide concentrations, this effect being less pronounced in tumour; (iii) conversely, that two apparent affinities for MgATP(2-) merged into one at high Mg(2+) concentration; (iv) kinetically, that this relationship is of the mixed con-competitive type, which is consistent with a role for Mg(2+) as a requisite activator, and binding occurring in non-ordered sequence. Analysis of the Ca(2+) effects showed: (i) competition with Mg(2+) at the metal site (K(i) 20mum for bone and 40mum for tumour); (ii) that relative to the substrate the inhibition was uncompetitive, i.e. velocity decreased and affinity increased proportionally, which is consistent with Ca(2+) binding after substrate binding. These findings support the existence of interacting enzyme complexes, losing co-operativity at increased enzyme activity. They also indicate a potential physiological role for Ca(2+) in enzyme regulation and point to quantitative differences between bone and tumour with regard to these properties.