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.     

Kinetics and mechanism of acetohydroxy acid synthase isozyme III from Escherichia coli

Acetohydroxy acid synthase (AHAS, EC 4.1.3.18) isozyme III from Escherichia coli has been studied in steady-state kinetic experiments in which the rates of formation of acetolactate (AL) and acetohydroxybutyrate (AHB) have been determined simultaneously. The ratio between the rates of production of the two alternative products and the concentrations of the substrates pyruvate and 2-ketobutyrate (2KB) leading to them, R, VAHB/VAL = R[( 2KB]/[pyruvate]), was found to be 40 +/- 3 under a wide variety of conditions. Because pyruvate is a common substrate in the reactions leading to both products and competes with 2-ketobutyrate to determine whether AL or AHB is formed, steady-state kinetic studies are unusually informative for this enzyme. At a given pyruvate concentration, the sum of the rates of formation of AL and AHB was nearly independent of the 2-ketobutyrate concentration. On the basis of these results, a mechanism is proposed for the enzyme that involves irreversible and rate-determining reaction of pyruvate, at a site which accepts 2-ketobutyrate poorly, if at all, to form an intermediate common to all the reactions. In the second phase of the reaction, various 2-keto acids can compete for this intermediate to form the respective acetohydroxy acids. 2-Keto acids other than the natural substrates pyruvate and 2-ketobutyrate may also compete, to a greater or lesser extent, in the second phase of the reaction to yield alternative products, e.g., 2-ketovalerate is preferred by about 2.5-fold over pyruvate. However, the presence of an additional keto acid does not affect the relative specificity of the enzyme for pyruvate and 2-ketobutyrate; this further supports the proposed mechanism. The substrate specificity in the second phase is an intrinsic property of the enzyme, unaffected by pH or feedback inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)     

The specificity of fumarate as a switching factor of the bacterial flagellar motor

Fumarate restores to flagella of cytoplasm-free, CheY- containing envelopes of Escherichia coli and Salmonella typhimurium the ability to switch from one direction of rotation to another. To examine the specificity of this effect, we studied flagellar rotation of envelopes which contained, instead of fumarate, one of its analogues. Malate, maleate and succinate promoted switching, but to a lesser extent than fumarate. These observations were made both with wild-type envelopes and with envelopes of a mutant which lacks the enzymes succinate dehydrogenase and fumarase, indicating that the switching-promoting activity of the analogues was not caused by their conversion to fumarate. Aspartate and lactate did not promote switching. Using strains defective in specific enzymes of the tricarboxylic acid cycle and lacking the cytoplasmic chemotaxis proteins as well as some of the chemo-taxis receptors, we demonstrated that, in intact bacteria, unlike the situation in envelopes, fumarate promoted clockwise rotation via its metabolites acetyl phosphate and acetyladenylate, but did not promote switching (presumably because of the presence of cytoplasmic fumarate). All of the results are consistent with the notion that fumarate acts as a switching factor, presumably by lowering the activation energy of switching. Thus fumarate and some of its metabolites may serve as a connection point between the bacterial metabolic state and chemotactic behaviour.     

A mutant of Bacillus thuringiensis var. israelensis (B.t.i.) resistant to antibiotics

Summary PST, a spontaneous mutant of Bacillus thuringiensis var. israelensis (B.t.i.) resistant to penicillin, streptomycin and tetracycline was isolated by serial selections. In the absence of antibiotics it showed genetic stability for 16 generations. Mosquito larvicidal activity of PST was similar to that of B.t.i., its parental strain. It also maintained the specific antigenicity of B.t.i. although its rate of growth was somewhat lower, a generation time of 55 min for PST vs. 38 min for B.t.i. Cell concentration plays a major role in the phenomenon of PST resistance to penicillin.This antibiotic resistant mutant of b.t.i. provides us with an efficient tool to trace B.t.i. among the indigenous bacteria present in septic habitats in the field as well as inside the larval gut.     

Enhanced binding of a 95 kDa protein to p53 in cells undergoing p53-mediated growth arrest.

To explore the biochemical functions of p53, we have initiated a search for cellular p53-binding proteins. Coprecipitation of three polypeptides was observed when cell lines overexpressing a temperature-sensitive (ts) p53 mutant were maintained at 32.5 degrees C (wild-type p53 activity, leading to growth arrest) but not at 37.5 degrees C (mutant p53 activity). One of these three proteins, designated p95 on the basis of its apparent molecular mass, was highly abundant in p53 immune complexes. We demonstrate herein that p95 is a p53-binding protein, which exhibits poor p53-binding in cells overproducing several distinct mutant p53 proteins. Yet, p95 associates equally well with both the wild-type (wt) and the mutant conformations of the ts p53 in transformed cells growth-arrested at 32.5 degrees C. On the basis of our findings we suggest that wt p53 activity increases p53-p95 complex formation and that such interaction may play a central role in p53 mediated tumour suppression.     

The pattern and meaning of neighbor relations in high-rise housing in Israel

This paper deals with the behavioral pattern of neighbor relations and their relationships to the subjective attitudes and expectations of the residents. The sample was 318 middle-class women living in eight-to 20-story buildings. The findings indicate that respondents so desiring were able to develop active social ties with their neighbors. Moreover, they interacted with neighbors despite the fact that the majority had opportunities for alternative social relations. The distinction between localized and nonlocalized high-rise residents does not seem meaningful in this case. Actual social ties with neighbors were related to norms and expectations regarding neighbor relations. Despite active neighboring, respondents did not have difficulties obtaining privacy.     

The synergistic tumoricidal activity of anticancer drugs and oxidative burst-triggered macrophages

Summary The anticancer drugs adriamycin (ADR) and actinomycin D (AMD) were tested for their effect on the oxidative burst (OB) of mouse peritoneal macrophages (MPM) and on the killing of tumor cells by OB-stimulated MPM. The oxidative burst of MPM determined by hydrogen peroxide (H₂O₂) production was severely impaired by ADR (10 g/ml) and AMD (40 g/ml) after a 1 h treatment and by lower concentrations of the drugs following a 24 h treatment. The toxicity of the drugs against MPM was comparable to their effect on EL4 cells. Pretreatment of EL4 and TLX-9 tumor cells with sublethal amounts of ADR for 4 h rendered the cells sensitive to the cytotoxic effect of OB-stimulated MPM which were otherwise unable to kill these cells. It seems that anticancer drugs and OB-stimulated macrophages can cooperate in the destruction of tumor cells in vitro.     

Parameters affecting the in vitro maturation of human monocytes to macrophages

In vitro maturation of human monocytes to macrophages was characterized by morphological criteria, cell size and lysosomal enzymes activity. Purified populations of monocytes were maintained in culture at either adherent or nonadherent conditions and their maturation to macrophages was observed in both cases. The addition of external factors such as hydrocortisone and vitamin D3 inhibited monocyte maturation. In the absence of external factors, nonadherent monocytes were inhibited in their maturation for up to 10 days when plated at crowded cell concentrations. In addition, the presence of human serum in the culture media had a higher inhibitory activity than similar concentrations of fetal calf serum. Supernates from crowded macrophages were also inhibitory for monocyte maturation. We suggest the possibility that cell crowding, as well as soluble factors found in the serum and probably secreted by macrophages, participate in the regulation of monocyte development by inhibiting their maturation. Once released from this inhibitory signal or environment, the monocytes mature to macrophages.     

Acetohydroxy Acid Synthase Activity in Chlorella emersonii under Auto- and Heterotrophic Growth Conditions

Acetohydroxyacid synthase (AHAS) activity was studied in the green unicellular alga Chlorella emersonii. This activity and its regulation was compared in the algae grown autotrophically and heterotrophically on glucose in the dark. No evidence for the existence of more than one enzyme was found. The activity in crude extracts from either heterotrophically or autotrophically grown cells showed a K_m for pyruvate of 9 millimolar, a 22-fold preference for 2-ketobutyrate over pyruvate as the second substrate, 50% inhibition by 0.5 millimolar valine, and 50% inhibition by 0.3 micromolar sulfometuron methyl (SMM). Spontaneous mutants of the alga resistant to SMM were isolated, which appeared to be single gene mutants containing SMM-resistant AHAS activity. Hence, AHAS appears to be the sole direct target site of SMM in C. emersonii. The fact that the mutants had equivalent SMM resistance under auto- and heterotrophic conditions further supports the conclusion that the same enzyme functions under both physiological regimes. The addition of valine and isoleucine leads to partial relief of SMM inhibition of biomass increase, but not of SMM inhibition of cell division.     

The role ofSaccharomyces cerevisiae Cdc40p in DNA replication and mitotic spindle formation and/or maintenance

Successful progression through the cell cycle requires the coupling of mitotic spindle formation to DNA replication. In this report we present evidence suggesting that, inSaccharomyces cerevisiae, theCDC40 gene product is required to regulate both DNA replication and mitotic spindle formation. The deduced amino acid sequence ofCDC40 (455 amino acids) contains four copies of a β-transducin-like repeat. Cdc40p is essential only at elevated temperatures, as a complete deletion or a truncated protein (deletion of the C-terminal 217 amino acids in thecdc40-1 allele) results in normal vegetative growth at 23°C, and cell cycle arrest at 36°C. In the mitotic cell cycle Cdc40p is apparently required for at least two steps: (1) for entry into S phase (neither DNA synthesis, nor mitotic spindle formation occurs at 36°C and (2) for completion of S-phase (cdc40::LEU2 cells cannot complete the cell cycle when returned to the permissive temperature in the presence of hydroxyurea). The role of Cdc40p as a regulatory protein linking DNA synthesis, spindle assembly/maintenance, and maturation promoting factor (MPF) activity is discussed.