The inhibition of yeast alcohol dehydrogenase by borate

6 Final Report on the Safety Assessment of Sodium Borate and Boric Acid

Sodium Borate and Boric Acid are used in cosmetics as preservatives, antiseptics, water softeners, pH adjusters, emulsifiers, neutralizes, stabilizers, buffers, or viscosifiers.

Investigators have reported that Sodium Borate and Boric Acid are poorly absorbed through intact skin; however, both compounds are absorbed through abraded, denuded, or burned skin. In a 90-day dermal toxicity study, Boric Acid (25-200 mg/kg/day) was nonirritating and nontoxic when applied to the intact skin of rabbits. Sodium Borate and Boric Acid were relatively nontoxic when tested orally in animals.

A 5% Sodium Borate in water solution was mildly or moderately irritating to the skin of rabbits and guinea pigs, and practically nonirritating when instilled in rabbits' eyes. Acute studies indicated that, at 10% in water, Boric Acid was mildly or moderately irritating to the skin of rabbits and guinea pigs.

Sodium Borate or Boric Acid in the diet of rabbits and rats caused growth retardation. Doses of up to 1.06 g/kg/day Sodium Borate in the diet of male rats exerted toxic effects on the gonads as well as infertility.

Boric Acid was nonmutagenic in the Ames test. Boric Acid induced reduced eye phenocopies and lumpy chromosomal inclusions in Drosophila melano-gaster. Limited carcinogenic and teratogenic studies did not indicate a statistically significant effect.

In clinical studies, cosmetic formulations containing up to 3.2% Sodium Borate were nonirritating to moderately irritating and nonsensitizing when applied to human skin. Formulations containing up to 2.4% Boric Acid were moderately irritating and practically nonirritating. Photopatch testing of formulations containing 1.1% or 1.7% Sodium Borate were negative.

Based on the increased absorption of Boric Acid by damaged skin as compared to intact skin, as well as the testicular atrophy observed in experimental animals, the Panel concluded that Sodium Borate and Boric Acid, in concentrations ≤ 5%, are safe as cosmetic ingredients when used as currently recommended; however, cosmetic formulations containing free Sodium Borate or Boric Acid at this concentration should not be used on infant or injured skin.

Boric acid inhibits adenosine diphosphate-ribosyl cyclase non-competitively

Adenosine diphosphate-ribosyl cyclase (ADP-ribosyl cyclase) is a ubiquitous enzyme in eukaryotes that converts NAD+ to cyclic-ADP-ribose (cADPR) and nicotinamide. A quantitative assay for cADPR was developed using capillary electrophoresis to separate NAD+, cADPR, ADP-ribose, and ADP with UV detection (254 nm). Using this assay, the apparent Km and Vmax for Aplysia ADP-ribosyl cyclase were determined to be 1.24+/-0.05 mM and 131.8+/-2.0 microM/min, respectively. Boric acid inhibited ADP-ribosyl cyclase non-competitively with a Ki of 40.5+/-0.5 mM. Boric acid binding to cADPR, determined by electrospray ionization mass spectrometry, was characterized by an apparent binding constant, KA, of 655+/-99 L/mol at pH 10.3.

Use of phenylboronic acids to investigate boron function in plants. Possible role of boron in transvacuolar cytoplasmic strands and cell-to-wall adhesion

The only defined physiological role of boron in plants is as a cross-linking molecule involving reversible covalent bonds with cis-diols on either side of borate. Boronic acids, which form the same reversible bonds with cis-diols but cannot cross-link two molecules, were used to selectively disrupt boron function in plants. In cultured tobacco (Nicotiana tabacum cv BY-2) cells, addition of boronic acids caused the disruption of cytoplasmic strands and cell-to-cell wall detachment. The effect of the boronic acids could be relieved by the addition of boron-complexing sugars and was proportional to the boronic acid-binding strength of the sugar. Experiments with germinating petunia (Petunia hybrida) pollen and boronate-affinity chromatography showed that boronic acids and boron compete for the same binding sites. The boronic acids appear to specifically disrupt or prevent borate-dependent cross-links important for the structural integrity of the cell, including the organization of transvacuolar cytoplasmic strands. Boron likely plays a structural role in the plant cytoskeleton. We conclude that boronic acids can be used to rapidly and reversibly induce boron deficiency-like responses and therefore are useful tools for investigating boron function in plants.

Use of borate to control the 5'-position-selective microbial glucosylation of pyridoxine

Nearly 100% 5'-position selectivity of transglucosylation from maltodextrin to pyridoxine (PN) by cells of Verticillium dahliae TPU 4900 was observed when the reaction was carried out with borate. The same effect of borate was observed not only during synthesis of pyridoxine 5'-alpha-D-glucoside by partially purified enzyme of this strain but also during synthesis of this compound by other microorganisms and with other enzymes (alpha-glucosidase and cyclomaltodextrin glucanotransferase). The effect was thought to be caused by the formation of a borate complex with 3- and 4'-position hydroxyl groups of PN. A decrease in the formation of pyridoxine 5'-alpha-D-glucoside was observed in the reaction with borate, but this decrease was overcome by optimizing the pH and increasing the amount of cells in the reaction mixture.

Use of a polymer-bound flavin derivative for the rapid regeneration of NAD(P)+ from NAD(P)H in dehydrogenase systems

An aldehyde derivative of riboflavin was covalently attached by reductive alkylation to soluble polycationic supports. The flavopolymers so obtained were stable under operational conditions. The catalytic efficiency towards oxidation of NADH by these flavopolymers was demonstrated, and the kinetic parameters (Km and kcat) revealed an overall catalytic efficiency (kcat/Km) 185-fold greater compared to riboflavin. Various factors affecting the chemical regeneration of NAD+ from NADH such as pH, ionic strength, nature of the buffer etc. were studied. The most interesting result was the highly favourable influence of borate ions which increased the reaction rate by a factor 2-4 compared to the other buffers. The flavopolymers are very effective for in situ recycling of NAD(P)+. With up to 300-fold NADH----NAD+ conversions for the system using yeast alcohol dehydrogenase and up to 1500-fold NADPH----NADP+ regenerations for the system using glucose-6-phosphate dehydrogenase. These flavopolymers are superior to previous chemical recycling systems.