Nucleotide substrate channeling

Most of the precursor pools turnover within a few minutes [12], and the size of various sugar nucleotide pools have been determined by methods with different reliability. But even with the best methods, it is hard to interpret the measured numbers and translate them into a clear picture because the relative distribution of the precursors in the cytosol and Golgi is not known. The average cellular concentration of sugar nucleotide precursors may not be very meaningful, since cytosol is an operational definition (100,000 g supernatant) that may not detect compartments of cytoplasmic organization or substrate channeling. 

The skeletal muscle Ca channels also can be phosphorylated in vitro by a protein kinase endogenous to the T-tubule membranes [111,115]. This kinase is neither Ca - nor cyclic nucleotide-dependent [115], and is interesting in that it phosphorylates primarily the P subunit while the ai subunit is a poor substrate. However, the amount of this kinase that co-purifies with the T-tubule membranes is variable, and consequently, very few studies have been performed. So far, only low levels of phosphorylation have been obtained (no more than 0.2 mol phosphate/ mol P subunit) and no functional effects of this phosphorylation have been observed in reconstitution studies. 

A variety of guanine nucleotide binding proteins (G-proteins) involved with the regulation of adenylate cyclase activity and transducin in the retina (Section 2.3.1) are substrates for ADP-ribosylation. Cholera toxin and E. coli enterotoxin LT ADP-ribosylate, and hence activate, the stimulatory G-protein of adenylate cyclase, whereas pertussis toxin ADP-ribosylates, and inactivates the inhibitory G-protein of adenylate cyclase. The result of ADP-ribosylation by either mechanism is increased adenylate cyclase activity, and an increase in intracellular cAMP and the opening of membrane calcium channels. Again, there are endogenous ADP-ribosyltransferases that modify the same G-proteins, but in a controlled manner (Moss et al., 1997, 1999). 

The increase in oxidizable substrates (e.g., glucose and G-6-P) enhances ATP production, thereby inhibiting an ATP-sensitive K channel. This decrease in K conductance causes to rise, opening a voltage-sensitive Ccf channel Ccft then acts as the insulin secretagogue. The ATP-sensitive K channel in /3 cells is an octamer composed of four Kir 6.2 and four SURl subunits. Both subunits contain nucleotide-binding domains Kir 6.2 appears to mediate the inhibitory response to ATP SURl binds ADP, the channel activator diazoxide, and the channel inhibitors (and promoters of insulin secretion) sulfonylureas and meglitinide. 

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