Binding of coenzyme and substrate and coenzyme analogues to 6-phosphogluconate dehydrogenase from sheep liver. An X-ray study at 0.6 nm resolution

The analogues of the coenzyme NADP+, nicotinamide--8-bromo-adenine dinucleotide phosphate (Nbr8ADP+) and 3-iodopyridine--adenine dinucleotide phosphate (io3PdADP+), were prepared. Nbr8ADP+ was found to be active in the hydrogen transfer adn io3PdADP+ is a coenzyme competitive inhibitor for 6-phosphogluconate dehydrogenase. The binding of NADP+, NADPH and NADPH together with 6-phosphogluconate as well as that of both analogues to crystals of the enzyme 6-phosphogluconate dehydrogenase has been investigated at 0.6-nm resolution using difference electron density maps. The molecules bind in a similar position in a cleft in the enzyme subunit distant from the dimer interface. The orientation of the coenzyme in the site has been determined from the io3PdADP+ -NADP+ difference density. The ternary complex difference density extends beyond that of the nicotinamide moiety of the coenzyme and tentatively indicates substrate binding. No clear identification of the bromine atom of Nbr8ADP+ can be made. However, the analogue is bound more deeply in the cleft than is NADP+. The NADPH density is the most clearly defined and has thus been used to fit a molecular model using an interactive graphics system, checking for preferred geometry. A possible conformation is presented which is significantly different from that of NAD+ in the lactate dehydrogenase ternary complex.

An examination of potential matrices for the affinity chromatography of NADP+-linked dehydrogenases with special reference to 6-phosphogluconate dehydrogenase

1. 6-phosphogluconate dehydrogenase from sheep liver has been purified 350-fold by affinity chromatography with a final specific activity of 18 micronmol of NADP+/reduced min per mg of protein and an overall yield of greater than 40%. 2. A systematic investigation of potential ligands has been carried out: these included 6-phosphogluconate and NADP+, pyridoxal phosphate and several immobilized nucleotides. The results indicate that NADP+ is the most suitable ligand for the purification of 6-phosphogluconate dehydrogenase. 3. The effects of pH and alternative eluents have been examined in relation to the parameters known to affect the desorption phase of affinity chromatography; careful manipulation of the elution conditions permitted the separation of glucose 6-phosphate dehydrogenase, glutathione reductase and 6-phosphogluconate dehydrogenase from sheep liver on NADP+-Sepharose 4B. 4. A large-scale purification scheme for 6-phosphogluconate dehydrogenase is presented that uses the competitive inhibitors inorganic pyrophosphate and citrate as specific eluents.

Point Visibility Meter: a forward scatter instrument for the measurement of aerosol extinction coefficient

A new instrument for the measurement of atmospheric extinction coefficient-the Point Visibility Meter (PVM)-is introduced and its principle of operation discussed. The instrument measures the amount of light scattered at a particular forward scattering angle by haze or fog; the extinction coefficient sigma is then found from the empirical law relating sigma to the scatter at this angle. Detailed calculations of aerosol scattering properties using Mie theory are given to support this empirical law. Error estimates based on these calculations indicate rms errors of less than +/-12% for a range of sigma values 2-60 km(-1). Field trials of the instrument against standard transmissometers confirm these theoretical findings and show a linearity of instrumental output with extinction coefficient, with an rms error of +/-11.7%.

Modification toward dominance of a recessive lethal in the mouse

The effect of the recessive siren (srn) gene singly in the presence of one Danforth's short tail (Sd) gene was to modify the expression of srn towards dominance with 46 percent penetrance. All mice that could be classified as sirens from srn/+ x Sd/+ matings had the Sd type short tail, indicating that interaction between Sd and srn was the principal cause for modification towards dominance of srn. External expressivity was variable in Sd srn pups, with true sirens, semi-sirens and apposed leg pups produced, each traceable to pelvic abnormalities. Variation in the urogenital system was similar to that found in siren pups with the srn/srn genotype.

Structure-function relationships in lactate dehydrogenase

The binding of coenzyme and substrate are considered in relation to the known primary and tertiary structure of lactate dehydrogenase (EC 1.1.1.27). The adenine binds in a hydrophobic crevice, and the two coenzyme phosphates are oriented by interactions with the protein. The positively charged guanidinium group of arginine 101 then folds over the negatively charged phosphates, collapsing the loop region over the active center and positioning the unreactive B side of the nicotinamide in a hydrophobic protein environment. Collapse of the loop also introduces various charged groups into the vicinity of the substrate binding site. The substrate is situated between histidine 195 and the C4 position on the nicotinamide ring, and is partially oriented by interactions between its carboxyl group and arginine 171. The spatial arrangements of these groups may provide the specificity for the L-isomer of lactate.