Phosphate turnover

A. Laisk and D. A. Walker, Control of phosphate turnover as a rate limiting factor and possible source of oscillations in photosynthesis, a mathematical model. Proc. R. Soc. London B 227,... 

Fey SJ, Larsen PM, Cells JE. (1983) Evidence for coordinated phosphorylation of keratins and vimentin during mitosis in transformed human amnion cells. Phosphate turnover of modified proteins. FEBS Lett 157, 165-9. 

Schiff base, or quinoidal intermediates do not appreciably affect the rate of the a reaction.90113 (2) L-serine and amino acids such as O-methyl-L-serine that form ES III do stimulate the a reaction.113 (3) The rate of indole-3-glycerol phosphate turnover is roughly correlated with the rate of formation of ES III for each of the amino acids.90-113 (4) The kinetics of the lag in cleavage of indole-3-glycerol phosphate and synthesis of L-tryptophan under single turnover conditions correspond to the rate of ES III formation... 

Furthermore, phosphoinositide/inositol phosphate turnover plays an important role in the production of secondary messengers during abiotic stress in plants. Within a few seconds, hyperosmotic shock induced a rapid and transient increase of Ins( 1,4,5)P3 in Arabidopsis cultured cells. This fact strongly suggests the PI turnover system functions near an osmosensor however, the molecule that functions as the osmosensor has not been identified, except in yeast and bacteria. In the near future, we hope to determine which molecule can activate the PI turnover system in response to hyperosmotic stress in plants. 

Srivastava, A., Pines, M., and Jacoby, B., 1989, Enhanced potassium uptake and phosphatidylinositol-phosphate turnover by hypertonic mannitol shock. Physiol. Plant 11 320-325. 

The fi O-Pj system has also recently been applied to phosphates associated with ferric iron oxyhydroxide precipitates in submarine ocean ridge sediments (Blake et al., 2000, 2001). The gi O-Pj signature of phosphate associated with these authigenic Fe-oxyhydroxide precipitates indicates microbial phosphate turnover at elevated temperatures. The latter observation suggests that phosphate oxygen isotopes may be useful biomarkers for fossil hydrothermal vent systems. On the basis of this work, Blake et al. (2001) also hypothesize that authigenic phases extant on other planets may retain imprints of primitive biospheres, in the form of detectable and diagnostic fi O-Pj composition, imparted by biochemical, enzymatic processes. 

Hokin, L. E. and Hokin, M. R. (1955) Effects of acetylcholine on the phosphate turnover in phospholipids of brain cortex in vitro. Biochim. biophys. Acta., 16, 229-237. 

L2. Lahiri, S., and Banerjee, S., Carbohydrate metabolism and phosphate turnover rate in scorbutic guinea pigs. Proc. Soc. Exptl. Biol. Med. 93, 557-560 (1956). 

The rates of soil phosphate reactions also may differ from the rates of phosphate uptake by plants and of phosphate release by organic matter decay. This phosphate turnover would further upset soil phosphate equilibria. If a steady state (concentration is constant with time) existed between the soil and dissolved phosphate ions, it might be described by a reaction such as... 

C. Rate of Phosphate Turnover on Myosin LC20 during Contraction... 

Other results suggest a lower rate of phosphate turnover on LC20. Kitazawa et al. (1991) measured phosphatase activity in a-toxin-permeabilized portal vein from the rate of dephosphorylation in rigor solution with inhibited myosin light chain kinase (MLCK) activity. Their value was 0.017 s i at 15°C, which, corrected for temperature (Mitsui et al., 1994), is about nine times smaller than the value of Butler et al. Reconciliation of these findings will require additional studies. However, the conditions of the experiments were quite different and it should also be pointed out that in the experiments of Butler et al. (1994), only 76% of the light chain phosphate turned over at the fast rate, al-... 

Dephosphorylation. In vivo, phosphoprotein phosphatases (phos-phoprotein phosphohydrolase EC 3.1.3.16) participate in the regulation of phosphate turnover and in rapid reversal of the phosphorylating reactions, yet there is very little information on this important group of enzymes. Alkaline (104) and acid (105) phosphatases of milk can hydrolyze the phosphate groups of phosphoserine residues in the caseins. E. coli alkaline phosphatase dephosphorylates many phosphoproteins (106,107). A phosphoprotein phosphatase of liver dephosphorylates phosphorylated histones and protamines but has little or no activity on casein or phosvitin (108). The 60-fold purified enzyme had an apparent Km for dephosphorylation of histone I (FI) of 2 X 10"5M and a pH optimum of 7-8. Histone phosphatase activity was detected in all eukaryotic cells examined, but it was not found in the extracts of several prokaryotes. 

A great deal more needs to be known about these important phosphoprotein phosphatases and the in vivo role they may play in the regulation of cellular activity and phosphate turnover. An immobilized enzyme system for the phosphorylation of proteins has been described (108a). 

Obviously this work did not cover the whole field, and gaps were left. However, the work indicates clearly that no study of the distribution of P and the kinetics of phosphate turnover in the vitamin D-free body can be conducted with the expectation of getting a deeper understanding, if necessary consideration is not given to the amount of P supplied to the blood stream per unit of time. 

The second question concerns the fact that insulin action involves not only effects on glucose uptake, but other effects such as changes in phosphate turnover (Stadie, 1954) and the cellular accumulation of potassium (Verzdr, 1952), as well as effects on protein synthesis (Bouckaert and de Duve, 1947). Are these changes simply the secondary consequence of a primary insulin action upon cell permeability to glucose We do not know. Despite the appeal of a single unitary mechanism for insulin action, the possibility of direct effects of insulin on events other than glucose transfer cannot be ruled out. 

---