Determination of products of acetohydroxy acid synthase by the colorimetric method, revisited

The enzyme acetohydroxy acid synthase (AHAS, EC 4.1.3.18) catalyzes two competing reactions of physiological importance: condensation of two molecules of pyruvate to form acetolactate (AL) or condensation of pyruvate and 2-ketobutyrate to form acetohydroxybutyrate (AHB). The activity of AHAS is most frequently analyzed using the Westerfeld method, in which the acetoin formed upon decarboxylation of AL is determined by colorimetric reaction with creatine and alpha-naphthol. However, there has been confusion as to the interpretation of the results of this assay in the presence of both substrates, conditions which lead to formation of both AL and AHB. By applying this assay to enzymatically prepared samples of AL and AHB which have also been analyzed by two other independent methods, we show here that the color yield for AHB in the commonly used assay is 35-40% that for equivalent amounts of acetoin or AL. The relative color yield is not significantly affected by varying the time or temperature of various steps in the color-forming reaction. This information could in principle be used, together with an independent specific assay for AHB, to determine the composition of an AHAS product mixture; it would, however, be less accurate than a simultaneous chromatographic method.     

Solving the N-Queens problem with a binary Hopfield-type network

The application of a discrete Hopfield-type neural network to solving the NP-Hard optimization problem — the N-Queens Problem (NQP) — is presented. The applied network is binary, and at every moment each neuron potential is equal to either 0 or 1. The network can be implemented in the asynchronous mode as well as in the synchronous one with n parallel running processors. In both cases the convergence rate is up to 100%, and the experimental estimate of the average computational complexity is polynomial. Based on the computer simulation results and the theoretical analysis, the proper network parameters are established. The behaviour of the network is explained.     

Subunit function in eukaryote cytochrome c oxidase. A mutation in the nuclear-coded subunit IV allows assembly but alters the function and stability of yeast cytochrome c oxidase.

Strains of the yeast Saccharomyces cerevisiae disrupted in YCOX4, the nuclear gene encoding cytochrome c oxidase subunit IV, do not assemble a functional or spectrally visible oxidase. We report the characterization of a yeast strain, RM1, expressing a mutated YCOX4 gene which is temperature sensitive for respiration at 37 degrees C, but incorporates cytochrome aa3 over all growth temperatures. The mutant enzyme is less stable than the wild type, with subunit IV readily proteolyzed without gross denaturation of the complex but with a concomitant loss of oxidase activity. When grown fermentatively at 37 degrees C, cytochrome c oxidase from the mutant strain had a turnover number of less than 3% of the normal complex, while Km values and subunit levels were comparable to normal. Thus alterations in subunit IV can perturb the enzyme structure and alter its catalytic rate, implying a role for this subunit in cytochrome c oxidase function as distinct from assembly.     

Effect of bilirubin and its photodegradation products on the respiration of mitochondria isolated from rat liver

In 0.05--0.1 mmol.l-1 concentration, bilirubin inhibits ADP-activated respiration of isolated liver mitochondria; it has no effect on respiration in the absence of ADP. Bilirubin-induced inhibition of respiration is not abolished by serum albumin, but bilirubin bound to serum albumin and the photodegradation products of bilirubin have no inhibitory effect.