Metal ion effects on target sites of modification in metallocarboxypeptidase B

Native carboxypeptidase B and its Co2+-substituted derivative were oxidized by the active-site-directed agent m-chloroperbenzoic acid. The following results were obtained a) In the cobalt enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Peptide or ester pseudo-substrates protected the cobalt enzyme but not the zinc enzyme against inactivation. b) Upon oxidation and formation of Co3+, cleavage of peptide bonds occurred in the cobalt enzyme but not in the zinc enzyme. Both enzymes retained their original metal content. c) Following oxidation of the enzymes, amino acid analysis revealed a modification of a methionyl residue in the zinc enzyme only; the cobalt enzyme, on the other hand, showed a modification of a histidyl residue. d) Peptide mapping of the enzymes after cleavage by cyanogen bromide indicated that two methionyl peptides were missing in the oxidized zinc enzyme. These peptides point to Met-64 as the site of modification. The peptide map of the oxidized cobalt enzyme was similar to that of the unmodified native (i.e., zinc) enzyme. These studies indicate that the specific metal ion present in the enzyme imposes certain structural and functional differences on the active site, leading to differing reactivities of specific amino acid residues and to a different alignment of the active-site-directed reagent in the two enzymes.     

Threshold for inhibition of Na, K-ATPase by ELF alternating currents.

Alternating currents can increase or decrease the ATP-splitting activity of the membrane enzyme Na,K-ATPase. Either change depends on the AC frequency, and the greatest effect appears to be in the ELF range at about 100 Hz. The threshold for enzyme inhibition by AC was determined, and it is estimated to be an internal electric field circa 5 microV/cm. The corresponding current-density threshold approximates 8 nA/cm2.     

The potential bioavailability of organic C,N, and P through enzyme hydrolysis in soils of the Mojave Desert

Increases in the growth rate of plants and microbes in the Mojave Desert in response to predicted increases in precipitation and CO₂ due to global climate change may induce nutrient limitations. This study was designed to measure the pool of potentially bioavailable nutrients in soils of the Mojave Desert. Soils were collected from shrub and interspace microsites and then subjected to amendment with buffered solutions of an excess of various enzymes. The products of each enzyme reaction were then measured and the maximum quantity of hydrolyzable substrates was calculated. In interspace and shrub microsite soils, respectively, 14.5 and 9.7% of the organic C in the form cellulose, 60.0–97.8% and 61.2–100.0% of the organic N in the form protein, and 44.0 and 57.5% of the organic P was hydrolyzable. There were significant differences between microsites for hydrolyzable substrate using all enzyme amendments, except protease. We propose that accumulations of hydrolyzable organic C, N, and P in the Mojave Desert could be a result of the persistently dry soil conditions often found in desert ecosystems and the immobilization of enzymes, which may result in low diffusivity of soil substrates and enzymes and, accordingly, little degradation of organic C, N, and P. Alternatively, rapid nutrient cycling and immobilization by soil microorganisms could account for accumulations of organic C, N, and P. Further refinement of the methods used in this study could lead to a valuable tool for the assessment of potential bioavailability of nutrients in a variety of soils.     

Erythrocyte membrane proteins: A modified gorter-grendel experiment

Summary The pressure-area isotherm and shear resistance of spectrin-actin monolayers indicate a close-packed structure at about 1.0 m²/mg protein. This surface area is equivalent to a thickness of about two monolayers at the erythrocyte membrane inner face. The maximum elasticity (lowest compressibility) occurs at 0.7 m²/mg protein, indicating the limit of reversible compression. The mechanical properties of the monolayers approximate those of the intact membrane, suggesting that the structures are similar and that these monolayers may account for many of thein vivo properties.