Irreversible inactivation of lactate racemase by sodium borohydride reveals reactivity of the nickel-pincer nucleotide cofactor

The nickel-pincer nucleotide (NPN) cofactor discovered in lactate racemase from Lactiplantibacillus plantarum (LarALp) is essential for the activities of racemases/epimerases in the highly diverse LarA superfamily. Prior mechanistic studies have established a proton-coupled hydride-transfer mechanism for LarALp, but direct evidence showing that hydride attacks the C4 atom in the pyridinium ring of NPN has been lacking. Here, we show that sodium borohydride (NaBH4) irreversibly inactivates LarALp accompanied by a rapid color change of the enzyme. The altered ultraviolet-visible spectra during NaBH4 titration supported hydride transfer to C4 of NPN, and the concomitant Ni loss unraveled by mass spectrometry experiments accounted for the irreversible inactivation. High resolution structures of LarALp revealed a substantially weakened C-Ni bond in the metastable sulfite-NPN adduct where the NPN cofactor is in the reduced state. These findings allowed us to propose a mechanism of LarALp inactivation by NaBH4 that provides key insights into the enzyme-catalyzed reaction and sheds light on the reactivity of small molecule NPN mimetics.

Assessment of physico-chemical changes in UHT milk during storage at different temperatures

This research communication describes enzymatic and physico-chemical changes during storage of UHT milk. The UHT milk sample was stored at 5 and 30°C for 4 months and analyzed regularly at intervals of one month. During storage of UHT milk, there was a significant (P < 0.001) increase in non-protein nitrogen, non-casein nitrogen, soluble calcium, soluble magnesium and proteolysis, while a significant (P < 0.001) decrease in pH was observed. There was a slight change in the particle size and zeta potential of casein micelles. Changes were more pronounced in milk samples stored at 30°C than in those stored at 5°C. During storage, there occurred changes in pH, viscosity, salt balance and nitrogenous components which adversely affected quality. It was concluded that the proteolysis led to the acidification which had a destabilizing effect on the milk.

Effects of wet corn distiller's grains with solubles and nonprotein nitrogen on feeding efficiency, growth performance, carcass characteristics, and nutrient losses of yearling steers12

Wet distiller's grains with solubles (WDGS) are a common by-product feedstuff generated by the grain-ethanol industry, and it is used extensively by the cattle feeding industry. Distillers grains are typically high in protein; however, the protein in WDGS has a low ruminal degradability, and thus may result in a deficiency of RDP in the diet even when dietary CP concentrations are high. Assessment of the RDP needs in diets containing WDGS is needed to aid the cattle feeding industry in managing feed costs and potential environmental issues. To that end, we conducted 2 feeding studies to evaluate the supplemental RDP requirements of beef cattle fed steam-flaked corn-based finishing diets. In Exp. 1, 525 yearling steers (initial body weight = 373 ± 13 kg) received treatments in a 2 × 3 + 1 factorial. Dietary factors included WDGS (15 or 30% of DM) and nonprotein N (NPN; 0, 1.5, or 3.0% of DM) from urea (0, 0.52, and 1.06%). The control diet without WDGS contained 3.0% NPN (1.06% urea) and cottonseed meal. Diets were formulated to have equal crude fat concentrations. Overall gain efficiency among steers fed 15% WDGS was greatest for 1.5% NPN and least for 0% NPN (P = 0.07, quadratic), whereas gain efficiency decreased linearly (P < 0.09) as NPN increased in the 30% WDGS diets. Dressing percent was greater (P < 0.01) for the Control diet than for 15 or 30% WDGS. In Exp. 2, 296 steer calves (initial BW = 344 ± 12 kg) were fed 1 of 4 experimental diets that included a Control diet without WDGS (contained 3% NPN from urea, and cottonseed meal) and 15% WDGS diets with either 1.50, 2.25, or 3.00% NPN (0.52, 0.78, and 1.04% urea, respectively, on a DM basis). Overall gain efficiency on either a live or carcass-adjusted basis was not different among treatments (P > 0.15). Dietary NPN concentration did not influence growth performance (P > 0.21). Increasing dietary WDGS concentration resulted in decreasing (P < 0.05) diet digestibility (determined with an internal marker) and increasing (P < 0.05) N volatilization losses (determined by diet and manure N:P ratio); however, the effects of NPN level on digestibility and N losses were somewhat inconsistent across experiments. Results suggest that optimum performance for cattle fed 15% WDGS occurred when the diet contained between 1.5 and 2.25% NPN. However, no supplemental NPN was needed to support optimum performance in diets containing 30% WDGS.

TEM Tomography of Pores with Application to Computational Nanoscale Flows in Nanoporous Silicon Nitride (NPN)

Silicon nanomembrane technologies (NPN, pnc-Si, and others) have been used commercially as electron microscopy (EM) substrates, and as filters with nanometer-resolution size cut-offs. Combined with EM, these materials provide a platform for catching or suspending nanoscale-size structures for analysis. Usefully, the nanomembrane itself can be manufactured to achieve a variety of nanopore topographies. The size, shapes, and surfaces of nanopores will influence transport, fouling, sieving, and electrical behavior. Electron tomography (ET) techniques used to recreate nanoscale-sized structures would provide an excellent way to capture this variation. Therefore, we modified a sample holder to accept our standardized 5.4 mm × 5.4 mm silicon nanomembrane chips and imaged NPN nanomembranes (50–100 nm thick, 10–100 nm nanopore diameters) using transmission electron microscopy (TEM). After imaging and ET reconstruction using a series of freely available tools (ImageJ, TomoJ, SEG3D2, Meshlab), we used COMSOL Multiphysics™ to simulate fluid flow inside a reconstructed nanopore. The results show flow profiles with significantly more complexity than a simple cylindrical model would predict, with regions of stagnation inside the nanopores. We expect that such tomographic reconstructions of ultrathin nanopores will be valuable in elucidating the physics that underlie the many applications of silicon nanomembranes.

Non-destructive analysis of the conformational differences among feedstock sources and their corresponding co-products from bioethanol production with molecular spectroscopy

The objective of this study was to determine the possibility of using molecular spectroscopy with multivariate technique as a fast method to detect the source effects among original feedstock sources of wheat and their corresponding co-products, wheat DDGS, from bioethanol production. Different sources of the bioethanol feedstock and their corresponding bioethanol co-products, three samples per source, were collected from the same newly-built bioethanol plant with current bioethanol processing technology. Multivariate molecular spectral analyses were carried out using agglomerative hierarchical cluster analysis (AHCA) and principal component analysis (PCA). The molecular spectral data of different feedstock sources and their corresponding co-products were compared at four different regions of ca. 1800-1725 cm(-1) (carbonyl CO ester, mainly related to lipid structure conformation), ca. 1725-1482 cm(-1) (amide I and amide II region mainly related to protein structure conformation), ca. 1482-1180 cm(-1) (mainly associated with structural carbohydrate) and ca. 1180-800 cm(-1) (mainly related to carbohydrates) in complex plant-based system. The results showed that the molecular spectroscopy with multivariate technique could reveal the structural differences among the bioethanol feedstock sources and among their corresponding co-products. The AHCA and PCA analyses were able to distinguish the molecular structure differences associated with chemical functional groups among the different sources of the feedstock and their corresponding co-products. The molecular spectral differences indicated the differences in functional, biomolecular and biopolymer groups which were confirmed by wet chemical analysis. These biomolecular and biopolymer structural differences were associated with chemical and nutrient profiles and nutrient utilization and availability. Molecular spectral analyses had the potential to identify molecular structure difference among bioethanol feedstock sources and their corresponding co-products.