Calmodulin inhibition

Hens started to molt and ceased laying. Feed intake decreased about 90%. Zinc concentrations in pancreas increased 7-fold, in liver 6-fold, kidney 3-fold, and were elevated in shell gland and yolk. High Zn levels in kidney reflect high Zn excretion rates high pancreatic Zn (410 mg Zn/kg FW) may suppress the release of insulin by calmodulin inhibition, and could account for the rapid cessation of lay (Verheyen etal. 1990). 

Corneliussen B, Holm M, Waltersson Y, Onions J, Hallberg B, Thornell A, Grundstrom T. Calcium/calmodulin inhibition of basic-helix-loop-helix transcription factor domains. Nature 1994 368 760-764. 

Cox, J.L. and S.D. Harrison, Jr. Correlation of metal toxicity with in vitro calmodulin inhibition. Biochem. 

Alteration of MLC phosphorylation can occur at four possible sites inhibition of Ca + binding to calmodulin, inhibition of calmodulin activation of MLCK, direct inhibition of MLCK catalytic activity, or activation of myosin light chain phosphatase (MLCP). Although inhibitors of MLCP, such as okadaic acid, are well characterized, there are no known specific activators of MLCP. 

Cox, D.E. Meinke, M.H. Edstrom, R.D. Mechanism of calmodulin inhibition of cAMP-dependent protein kinase activation of phosphorylation kinase. Arch. Biochem. Biophys., 259, 350-362 (1987)... 

Cox JL, Harrison SD Jr (1983) Correlation of metal toxicity with in vitro calmodulin inhibition. Biochem Biophys Res commun 115 106-111 Der R, Fahim Z, Yousef M, Fahim M (1977) Effects of cadmium on growth, sexual development and metabolism in female rats. Res Commun Chem Pathol Pharmacol 16 485-505... 

CALMODULIN INHIBITION A POSSIBLE PREDICTOR OF METAL-ION TOXICITY... 

Toxicity prediction, metal ions, calmodulin inhibition. 

Cox and Harrison (1983) have reported observations giving the relative inhibition of calmodulin activity, in vitro, caused by 13 divalent metal ions. The metal-ion concentration required to produce a 50% reduced activation of cyclic nucleotide phosphodiesterase with bovine brain calmodulin (1050) was chosen as a measure of calmodulin inhibition in vitro. On the basis of the 1050 values, the ranking for the effectiveness of the 13 divalent metal ions studied was Hg2 > Cu " > Pt > Cd = Be " > Zn2+ > Pb > Ba = Pd > Co > Ni " > Mn2" > Sr2. ... 

Our studies of the effects of metal ions on cell survival (Tan et al. 1984) and those of Cox and Harrison (1983) on calmodulin inhibition have in common the 10 divalent metal ions, Cd2 Hg, Cu ", Zn, Be, Ni, Pd ", Co, Mn ", and Sr2+. FIGURE 1 shows the values of log CE50 from our studies (Tan et al. 1984) plotted against log 1050 from Cox and Harrison (1983). TABLE 1 gives the numerical values of these quantities. The data are represented by the regression line... 

In addition, vinpocetine selectively inhibits a specific calcium, calmodulin-dependent cycHc nucleotide phosphodiesterase (PDF) isozyme (16). As a result of this inhibition, cycHc guanosine 5 -monophosphate (GMP) levels increase. Relaxation of smooth muscle seems to be dependent on the activation of cychc GMP-dependent protein kinase (17), thus this property may account for the vasodilator activity of vinpocetine. A review of the pharmacology of vinpocetine is available (18). 

Diltiazem inhibits calcium influx via voltage-operated channels and therefore decreases intracellular calcium ion. This decreases smooth muscle tone. Diltiazem dilates both large and small arteries and also inhibits a-adrenoceptor activated calcium influx. It differs from verapamil and nifedipine by its use dependence. In order for the blockade to occur, the channels must be in the activated state. Diltiazem has no significant affinity for calmodulin. The side effects are headache, edema, and dizziness. 

Anthrax toxin Lethal factor Lethal factor MEKs Endoprotease Increase in intracellular cAMP Inhibition of MAP-kinase pathways Calmodulin dependent adenylylcyclase... 

The anthrax toxin is a tripartite toxin and consists ofthe binding component protective antigen (PA), the lethal factor (LF), which is a metalloprotease, and the edema factor (EF), which is a calmodulin-dependent adenylyl-cyclase. Both enzyme components are translocated via PA into target cells. PA is activated by furin-induced cleavage and forms heptamers, which are similar to the binding components of C2 toxin and iota toxin. In the low pH compartment of endosomes, the heptamers form pores to allow translocation of LF and EF. LF cleaves six of the seven MEKs (MAPK-kinases) thereby inhibiting these enzymes. The functional consequence is the blockade of the MAPK pathways that control cell proliferation, differentiation, inflammation, stress response, and survival. Whether this is the reason for the LT-induced cell death of macrophages is not clear [1]. 

Calcium-dependent regulation involves the calcium-calmodulin complex that activates smooth muscle MLCK, a monomer of approximately 135 kDa. Dephosphorylation is initiated by MLCP. MLCP is a complex of three proteins a 110-130 kDa myosin phosphatase targeting and regulatory subunit (MYPT1), a 37 kDa catalytic subunit (PP-1C) and a 20 kDa subunit of unknown function. In most cases, calcium-independent regulation of smooth muscle tone is achieved by inhibition of MLCP activity at constant calcium level inducing an increase in phospho-rMLC and contraction (Fig. 1). 

Figure 6. A hypothetical scheme for the control of the number of active crossbridges in smooth muscle. Following the activation of a smooth muscle by an agonist, the concentrations of intermediates along the main route begins to build up transiently. This is shown by the thickened arrows. Also, cAMP is generated which is universally an inhibitor in smooth muscle. Cyclic AMP in turn combines with protein kinase A, which accounts for most of its action. The downstream mechanisms, however, are not well worked out and at least three possibilities are likely in different circumstances. First, protein kinase A is known to catalyze the phosphorylation of MLCK, once phosphorylated MLCK has a relatively lower affinity for Ca-calmodulin so that for a given concentration of Ca-calmodulin, the activation downstream is reduced. The law of mass action predicts that this inhibition should be reversed at high calcium concentrations. Other cAMP inhibitory mechanisms for which there is evidence include interference with the SR Ca storage system, and activation of a MLC phosphatase.

In in vitro experiments, caldesmon can inhibit the activation of myosin by actin and this inhibition can be reversed by calmodulin. Thus, there is a potentiality for... 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

The calmodulin-4Ca +-activated light chain kinase phosphorylates the hght chains, which then ceases to inhibit the myosin-F-actin intetaction. The contraction cycle then begins. 

Effect of protein-bound Ca TpC 4Ca antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) Calmodulin 4Ca activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin-myosin interaction (allows F-actin activation of ATPase). 

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