Cyclase

About: Cyclase is a research topic. Over the lifetime, 10162 publications have been published within this topic receiving 388566 citations.

Highly cooperative feedback control of retinal rod guanylate cyclase by calcium ions

Abstract: Visual excitation in retinal rod cells is mediated by a cascade that leads to the amplified hydrolysis of cyclic GMP (cGMP) and the consequent closure of cGMP-activated cation-specific channels in the plasma membrane1–3. Recovery of the dark state requires the resynthesis of cGMP, which is catalysed by guanylate cyclase, an axoneme-associated enzyme4–6. The lowering of the cytosolic calcium concentration (Cai) following illumination7–10 is thought to be important in stimulating cyclase activity11,12. This hypothesis is supported by the finding that the cGMP content of rod outer segments increases several-fold when Cai is lowered to less than 10 nM13–16. It is evident that cGMP and Cai levels are reciprocally controlled by negative feedback1,2. Guanylate cyclase from toad ROS is strongly stimulated when the calcium level is lowered from 10 μM to 10 nM, but only if they are excited by light17. We show here that the guanylate cyclase activity of unilluminated bovine rod outer segments increases markedly (5 to 20-fold) when the calcium level is lowered from 200 nM to 50 nM. This steep dependence of guanylate cyclase activity on the calcium level in the physiological range has a Hill coefficient of 3.9. Stimulation at low calcium levels is mediated by a protein that can be released from the outer segment membranes by washing with a low salt buffer. Calcium sensitivity is partially restored by adding the soluble extract back to the washed membranes. The highly cooperative activation of guanylate cyclase by the light-induced lowering of Cai is likely to be a key event in restoring the dark current after excitation.

Aluminum: a requirement for activation of the regulatory component of adenylate cyclase by fluoride.

Abstract: Activation of the purified guanine nucleotide-binding regulatory component (G/F) of adenylate cyclase by F- requires the presence of Mg2+ and another factor. This factor, which contaminates commercial preparations of various nucleotides and disposable glass test tubes, has been identified as Al3+. In the presence of 10 mM Mg2+ and 5 mM F-, AlCl3 causes activation of G/F with an apparent activation constant of approximately 1-5 muM. The requirement for Al3+ is highly specific; of 28 other metals tested, only Be2+ promoted activation of G/F by F-.

Forskolin: a unique diterpene activator of cyclic AMP-generating systems

Abstract: Forskolin, a diterpene of the labdane family, activates adenylate cyclase in broken cell preparations as well as in intact tissues. This activation does not require the guanine nucleotide regulatory subunit of the enzyme and probably occurs via an interaction with the catalytic subunit of adenylate cyclase. Activation of adenylate cyclase by forskolin results in marked increases in levels of intracellular cyclic AMP in a variety of eukaryotic cells. Low concentrations of forskolin which alone elicit small increases in intracellular cyclic AMP greatly potentiate hormonal activation of adenylate cyclase in a number of intact cells. Forskolin elicits cellular responses which have been proposed to be dependent o cyclic AMP as a second messenger. Forskolin, thus provides an invaluable tool for the investigation of the role of cyclic AMP in physiological responses to hormones, both through it direct activation of adenylate cyclase and through its ability to potentiate hormonal activation of adenylate cyclase.