Introduction of a phosphate at serine741 of the calmodulin-binding domain of the neuronal nitric oxide synthase (NOS-I) prevents binding of calmodulin

Binding kinetics of calmodulin with target peptides of three nitric oxide synthase isozymes

Efficient electron transfer from reductase domain to oxygenase domain in nitric oxide synthase (NOS) is dependent on the binding of calmodulin (CaM). Rate constants for the binding of CaM to NOS target peptides was only determined previously by surface plasmon resonance (SPR) (Biochemistry 35, 8742-8747, 1996) suggesting that the binding of CaM to NOSs is slow and does not support the fast electron transfer in NOSs measured in previous and this studies. To resolve this contradiction, the binding rates of holo Alexa 350 labeled T34C/T110W CaM (Alexa-CaM) to target peptides from three NOS isozymes were determined using fluorescence stopped-flow. All three target peptides exhibited fast k(on) constants at 4.5°C: 6.6×10(8)M(-1)s(-1) for nNOS(726-749), 2.9×10(8)M(-1)s(-1) for eNOS(492-511) and 6.1×10(8)M(-1)s(-1) for iNOS(507-531), 3-4 orders of magnitude faster than those determined previously by SPR. Dissociation rates of NOS target peptides from Alexa-CaM/peptide complexes were measured by Ca(2+) chelation with ETDA: 3.7s(-1) for nNOS(726-749), 4.5s(-1) for eNOS(492-511), and 0.063s(-1) for iNOS(507-531). Our data suggest that the binding of CaM to NOS is fast and kinetically competent for efficient electron transfer and is unlikely rate-limiting in NOS catalysis. Only iNOS(507-531) was able to bind apo Alexa-CaM, but in a very different conformation from its binding to holo Alexa-CaM.

Inhibitors of phosphodiesterase 5 (PDE 5) inhibit the nerve-induced release of nitric oxide from the rabbit corpus cavernosum

BACKGROUND AND PURPOSE

Nitrergic neurons are important for erectile responses in the corpus cavernosum and impaired signalling results in erectile dysfunction, today treated successfully by oral administration of the selective phosphodiesterase 5 (PDE 5) inhibitors sildenafil, tadalafil and vardenafil. Although the importance of nitrergic neurons in urogenital function has become evident, it has not been investigated if the PDE 5 inhibitors affect the nerve-induced release of nitric oxide (NO). In a previous study we found that the soluble guanylate cyclase (sGC)/cyclic guanosine 3',5'-monophosphate (cGMP) pathway might modulate nerve-induced release of NO in isolated cavernous tissue.

EXPERIMENTAL APPROACH

Electrical field stimulation (EFS 5 Hz, 40 V, 0.3 ms pulse duration, 25 pulses at intervals of 2 min) of rabbit isolated cavernous tissue elicited reproducible, nerve-mediated relaxations in the presence of scopolamine (10(-5) M), guanethidine (10(-5) M) and phenylephrine (3 x 10(-6) M). In superfusion experiments, nerve stimulation (20 Hz, 40 V, 1 ms) of the cavernous tissue evoked release of NO/NO2-, measured by chemiluminescence.

KEY RESULTS

Sildenafil, tadalafil and vardenafil decreased the muscular tone and prolonged the relaxations to nerve stimulation. The evoked release of NO decreased to 72+/-11%, 55+/-16% and 61+/-14% of control, respectively after addition of sildenafil, tadalafil or vardenafil (all 10(-4) M, n=6-8, p<0.05).

CONCLUSIONS AND IMPLICATIONS

Selective PDE 5 inhibitors influence the nerve-induced release of NO, probably via cGMP-mediated negative feedback. This negative feedback might explain why priapism is not seen during monotherapy with the PDE inhibitors.

Calcium/calmodulin-dependent protein kinase I inhibits neuronal nitric-oxide synthase activity through serine 741 phosphorylation

We demonstrate here that neuronal nitric-oxide synthase (nNOS) is phosphorylated and inhibited by a constitutively active form of Ca2+/calmodulin (CaM)-dependent protein kinase I (CaM-K I1-293). Substitution of Ser741 to Ala in nNOS blocked the phosphorylation and the inhibitory effect. Mimicking phosphorylation at Ser741 by Ser to Asp mutation resulted in decreased binding of and activation by CaM, since the mutation was within the CaM-binding domain. CaM-K I1-293 gave phosphorylation of nNOS at Ser741 in transfected cells, resulting in 60-70% inhibition of nNOS activity. Wild-type CaM-K I also did phosphorylate nNOS at Ser741 in transfected cells, but either CaM-K II or CaM-K IV did not. These results raise the possibility of a novel cross-talk between nNOS and CaM-K I through the phosphorylation of Ser741 on nNOS.

Target recognition of apocalmodulin by nitric oxide synthase I peptides

An increasing number of proteins are found that are regulated by the Ca(2+)-free state of calmodulin, apocalmodulin. Many of these targets harbor a so-called IQ motif within their primary sequence, but several target proteins of apocalmodulin lack this motif. We investigated whether the Ca(2+)-dependent calmodulin-binding site of nitric oxide synthase I could be transformed into a target site of apocalmodulin. Synthetic peptides representing the wild-type amino acid sequence and several peptides carrying mutations were studied by isothermal titration calorimetry and fluorescence spectroscopy. A single amino acid substitution of a negative charge to a positive charge can convert a classical Ca(2+)-dependent binding site of calmodulin into a target site for apocalmodulin. In addition, the introduction of hydrophobic amino acids increases the apparent binding affinity from the micromolar to the nanomolar range. Binding of wild-type and mutant peptides to Ca(2+)-calmodulin was enthalpically driven, and binding to apocalmodulin was entropically driven. Our data indicate that only a few selected amino acid positions in a calmodulin-binding site determine its Ca(2+) dependency.

Immunochemical characterization and measurement of neuronal type nitric oxide synthase in human neuroblastoma NB-OK-1 cell using novel anti-synthetic peptide antibody and specific immunoassay system

We developed a sensitive and specific immunoassay system for human neuronal nitric oxide synthase (hnNOS) using synthetic hnNOS(998-1024) peptide and anti-hnNOS(998-1024) antibody. The novel antibody and radioimmunoassay system revealed a typical nNOS protein in human neuroblastoma NB-OK-1 cell (160 kDa, 180 fmol/10(6) cells). The kinetic parameters of the enzyme were K(m)=4.88 microM and V(max)=4.34 pmol/min/mg protein for L-arginine. On incubation of NB-OK-1 cell for 24 h, betamethasone phosphate decreased both nNOS-immunoreactivity (nNOS-IR) and enzymatic activity in the cell dose-dependently. On the other hand, pituitary adenylate cyclase activating polypeptide(1-38) (PACAP38) increased both nNOS-IR and enzymatic activity at concentrations of 10(-10) and 10(-9) M, but inversely decreased both at 10(-7) M. These suggest the positive and negative implications of endogenous NO in proliferation and differentiation of the cell, which support mitogenic activity of NO generated by nNOS in the cell. The present findings also provided evidence that the quantitative change of nNOS protein controls the integrated activity of the enzyme in the cell and, in turn, substantiate the validity and reliability of the present immunoassay system for hnNOS and its practical usefulness.

Application of different surface plasmon resonance biosensor chips to monitor the interaction of the CaM-binding site of nitric oxide synthase I and calmodulin

Surface plasmon resonance biosensors depend on modified gold surfaces to allow immobilization of proteins or peptides for interaction analysis. We investigated sensor chip surfaces that differ in the geometry of the immobilization matrix: two contain a three-dimensional coupling matrix and two have a surface with immobilization sites on a two-dimensional plane. Properties of sensor chips were compared by studying the interaction of calmodulin with a peptide representing the calmodulin-binding site of nitric oxide synthase I. Apparent K(D) values were determined by three different procedures in order to apply tests for self-consistency. At low surface densities (5-8 fmol/mm(2)) on three of the four tested surfaces, estimated K(D) values were within one order of magnitude and similar to the value found in solution (K(D) = 1-3 nM). When immobilization densities were increased by one to two orders of magnitude, apparent association rate constants were less distorted on a flat carboxymethylated surface than on dextran-coated sensor chips.

Mutations at lysine 525 of inducible nitric-oxide synthase affect its Ca2+-independent activity

Calmodulin binding to inducible nitric-oxide synthase may play an important role in its Ca(2+)-independent activity. Studies of inducible nitric-oxide synthase chimeras containing the calmodulin binding sequence of neuronal or endothelial nitric-oxide synthases show that the calmodulin binding sequence of inducible nitric-oxide synthase is necessary but not sufficient for the Ca(2+)-independent activity. The mutations at lysine 525 located at the C terminus of the calmodulin binding sequence of inducible nitric-oxide synthase were examined for the effects on the Ca(2+)-independent activity with chimeras containing the oxygenase or reductase domains of inducible or neuronal nitric-oxide synthases. Results show that the Ca(2+)-independent binding of calmodulin is not solely responsible for maximal Ca(2+)-independent activity of inducible nitric-oxide synthase. Lysine 525 of inducible nitric-oxide synthase may also play an important role in coordinating the maximal Ca(2+)-independent activity.

RdgC/PP5-related phosphatases: novel components in signal transduction

A great variety of cellular functions are regulated by protein serine/threonine phosphatases (PP). This review summarises the current knowledge of the structural features, patterns of expression and involvement in signal transduction pathways of protein serine/threonine phosphatases related to PP5 and RdgC. Designated now as PP5/RdgC subfamily by P. T. W. Cohen in her 1997 study published in Trends in Biochemical Sciences, (Vol. 22, pp. 245-251), this heterogeneous group comprises phosphatases PP5/PPT, containing regulatory domains with tetratricopeptide repeats, RdgC/PPEF, which possess Ca2+-binding EF hand-type sites, and, recently discovered in plants, PP7. PP5 is ubiquitously expressed and appears to be a multifunctional phosphatase involved in a number of different signalling pathways. In contrast, expression of RdgC/PPEF phosphatases and PP7 is confined primarily to specialised sensory cells in animals and plants, respectively, which may be indicative of their more specialised roles in sensory signal transduction.