Santiago Gascón

alpha-Galactosidase from Saccharomyces carlsbergensis. Cellular localization, and purification of the external enzyme.

1. The alpha-galactosidase of Saccharomyces carlsbergensis in an inducible enzyme which is localized mainly outside the cell membrane and which is secreted into the culture medium in increasing amounts during the growth cycle. 2. The soluble form of alpha-galactosidase localized inside the cell appears to have the same characteristics as the external one, contrasting with the different forms found in the case of invertase. Although some activity is membrane-bound, this activity, when solubilized with detergent, has the same characteristics as the external form of the enzyme. 3. A procedure has been developed by which the enzyme has been purified using batch adsorption with DEAE-Sephadex and column chromatography in DEAE-Sephadex, DEAE-cellulose and Sephadex G-200, using the supernatant of a culture of Saccharomyces carlsbergensis grown in yeast/nitrogen base complemented with galactose. 4. The purified enzyme, which is homogeneous by chromatographic criteria and polyacrylamide gel electrophoresis, appears to be glycoprotein. 5. Invertase copurifies with the alpha-galactosidase but because of its lower stability, together with the fact that the synthesis of both enzymes can be controlled separately, it was possible to obtain preparations in which the contaminant activity was approximately 1%.

The marine toxin okadaic acid is a potent neurotoxin for cultured cerebellar neurons

The tumor promoter okadaic acid (OKA), is a marine toxin of algal origin, identified as a potent inhibitor of protein phosphatases 1 and 2A, and possibly enhancing calcium influx through voltage dependent calcium channels (VSSC). We now report that OKA at concentrations as low as 0.5 nM produced neurotoxicity, characterized first by the desintegration of the neurites and swelling of cell bodies, and later by cellular death. Non-neuronal cells viability and morphology were unaffected up to at least 5 nM OKA. Neurons sensitivity to the toxin changed with age in culture. Maximum neurotoxicity was observed in neurons at 9 DIC, when the OKA concentration producing half of the maximum reduction in neuronal survival (EC50) was approximately 0.65 nM. At 5 DIC or 19 DIC (EC50 approximately 2.5 nM and approximately 4.5 nM respectively), neurons appeared to be less sensitive to OKA. Neurotoxicity by OKA was not reduced by VSCC antagonists such as nifedipine and verapamil, nor by antagonists of excitatory aminoacid (EAA) receptors including APV, MK801 or CNQX. VSCC antagonists and EAA receptors antagonists fully protected from neurotoxicity induced by depolarization with KCl. These results suggest that OKA mechanism of neurotoxicity may not directly involve VSCC, endogenous EAA release and EAA receptors, but may depend upon other neurochemical events.

Catabolite inactivation of isocitrate lyase from Saccharomyces cerevisiae.

A reversible carbon catabolite inactivation step is described for isocitrate lyase from Saccharomyces cerevisiae. This reversible inactivation step of isocitrate lyase is similar to that described for fructose 1,6-bisphosphatase. Addition of 2,4-dinitrophenol, nystatin or glucose to cultures, grown in ethanol as carbon source, caused a rapid loss of the isocitrate lyase and fructose 1,6-bisphosphatase activities at pH 5.5 but not at pH 7.5. These results suggest that intracellular acidification and thus a cAMP increase is involved in the catabolite inactivation mechanism of both enzymes. From results obtained by addition of glucose to yeast cultures at pH 7.5 it was concluded that others factors than cAMP can play a role in the catabolite inactivation mechanism of both enzymes.

α-galactosidase (melibiase) from Saccharomyces carlsbergensis: Structural and kinetic properties

Abstract This report describes structural and kinetic properties of the purified α-galactosidase from Saccharomyces carlsbergensis . This galactosidase has many similar properties to other exocellular enzymes in yeast which have been reported. Its molecular weight of 300,000 is comparable; it has similar carbohydrate content (57%) and amino acid and carbohydrate composition. That is, 35% of its amino acid residues can be accounted for by threonine, serine, and aspartic acid. Its carbohydrate composition is primarily mannose (90–95%) with approximately 7% glucose and 1% glucosamine. The enzyme is very stable to both acidic and alkaline conditions as well as to heating to 50 °C. α-Galactosidase remains active after incubation with as much as 1% sodium dodecyl sulfate at 30 °C. However, the enzyme is denatured with urea and guanidine hydrochloride. The loss of activity is proportional to the urea concentration, the nondenatured enzyme being responsible for the remaining activity. Inactivation by urea is partially reversible. With urea or 60 °C heat denaturation, the enzyme dissociates into two types of subunits as revealed by polyacrylamide gel electrophoresis with sodium dodecyl sulfate. Thus, α-galactosidase is the first external enzyme from yeast in which an oligomeric structure is reported. The enzyme catalyzes the hydrolysis of p -nitrophenyl-α- d -galactoside, melibiose, and raffinose with similar pH optima and V max . However, the affinity is 20-fold lower for raffinose than for the other two substrates. Sugars having the same configuration in carbons 2, 3, and 4 as galactose competitively inhibit the enzyme.

Primary effects of yeast killer toxin

Abstract Killer toxin from Saccharomyces cerevisiae binds to sensitive cells immediately after addition to the cells. However, 50% mortality was obtained only after 40 minutes. Although it is thought that a lag phase is required for the killer to exert its action, we report experiments showing that the killer starts affecting the cell immediately after binding. Thus, shortly after addition the toxin was able to inhibit the transport of L-|;3H|; leucine as well as that of protons which are cotransported with this amino-acid or with histidine. Moreover, killer toxin inhibited the pumping of protons to the medium by cells which were actively metabolizing glucose. These effects were a function of the concentration of toxin used. The results suggest that killer toxin acts by affecting the electrochemical proton gradient across the plasma membrane of yeast.

Purification and characterization of a thermosensitive X-prolyl dipeptidyl aminopeptidase (dipeptidyl aminopeptidase yscV) from Saccharomyces cerevisiae

Abstract Dipeptidyl aminopeptidase yscV, a heat-labile enzyme with X-prolyl dipeptidyl aminopeptidase activity, was purified about 470-fold from a protoplast lysate of Saccharomyces cerevisiae. The purification procedure included solubilization of tonoplast-bound activity by the non-ionic detergent octyl-β- d -glucopyranoside, glycerol gradient centrifugation and preparative isoelectric focusing. Sodium dodecyl sulfate polyacrylamide gel electrophoresis resulted in a single band for which a molecular weight of 40 000 was calculated. The peptidase was most active at pH 7.0–7.5 with l -alanyl- l -proline -p- nitroanilide as substrate. Substrate specificity studies indicate that the purified enzyme specifically hydrolyzes peptide bonds involving the carboxyl group of prolyl residues penultimate to unprotected termini unless arginine is the N-terminal amino acid. However, X-Ala-arylamide structures are not attacked. The actinomycete inhibitors antipain, chymostatin and pepstatin had no effect on the enzyme activity, but 5 mM phenylmethylsulfonyl fluoride, an inhibitor of serine peptidases, completely inhibited dipeptidyl aminopeptidase yscV activity. Some heavy metals (Ni2+, Cd2+, Zn2+, Hg2+) at a concentration of 5·10−4 M were also found to be potent inhibitors of enzyme activity.

Some enzymatic properties of vacuolar alkaline phosphatase from yeast

Candida utilis alkaline phosphatase has been detected in vacuoles. Liberation of the vacuoles was carried out by protoplast disruption under isotonic conditions. The polybase DEAE-dextran was used to induce damage to the yeast plasmalemma. The vacuoles were purified by centrifugation on sorbitol-Ficoll gradients. Alkaline phosphatase from a purified fraction of vacuoles was characterized after gel filtration on Sephadex G-200. We have found 15 mU of enzyme activity per 108 vacuoles. This enzyme activity elutes on Sephadex G-200 at a volume-to-void-volume ratio of 1.65. The approximate molecular weight is 1.35×105. TheKm value forp-nitrophenyl-phosphate is 2.5×10−3 M. The pH for maximum activity is 8.9, and the enzyme is stable at pH values between 7.0 and 9.0. Rapid inactivation occurs at temperatures above 45°C. The enzyme catalyzes the hydrolysis of phosphomonoester bonds of a wide variety of molecules, especially polyphosphates. Thus, vacuolar polyphosphates are probably the natural substrate of this enzyme. Orthophosphate, arsenate, ethylenediaminetetraacetate, molybdate, and borate act as inhibitors. Fluoride is not an inhibitor, and the activity is not affected byp-hydroxymercuribenzoate. Some metal ions also affect the activity of vacuolar alkaline phosphatase. This may indicate that this enzyme is a metalloprotein.

Characteristics and regulation of a high conductance anion channel in GBK kidney epithelial cells.

Single anion selective channels have been studied in membrane patches of GBK cultured epithelial cells from bovine kidney. High conductance anion channels are exclusively observed in excised patches after holding them at membrane potentials larger than ±30 mV for seconds or minutes. Once activated, the channels show a steep voltage dependence, complex gating properties and multiple conductance levels. The major unit conductance is 300 pS (±28;n=12) in symmetrical chloride. The discrimination among different anions and between anions and cations is poor. The activity of the channels remains unchanged after addition of 10 mM EGTA to the cytoplasmic face of the membrane, but it is irreversibly inhibited by application of 1 mM 4-acetamido-4′-isothiocyano-2,2′-disulphonic acid stilbene (SITS). Neither permeabilization of the cell membrane to Ca2+ using the Ca2+ ionophore A23187, treatments which increase the cyclic AMP content of the cells, nor hypoosmotic shocks, activate high conductance anion channels in cell-attached patches in which the activity of the channels is subsequently demonstrated upon patch excision. These results indicate that high conductance anion channels with the same characteristics as those reported in different cell types are present in GBK cells and suggest that the physiological role for these molecular entities may be different from those previously postulated.

Xylose induced decrease in hexokinase PII activity confers resistance to carbon catabolite repression of invertase synthesis in Saccharomyces carlsbergensis

The relationship between the xylose induced decrease in hexokinase PII activity and the derepression of invertase synthesis in yeast is described. When xylose was added to cells growing in a chemostat under nitrogen limitation, the catabolic repression was supressed as shown by the large increase on invertase levels even if glucose remained high. The glucose phosphorylating-enzymes were separated by hydroxylapatite chromatography and it is shown that the treatment with xylose is accompanied by a loss of 98% hexokinase PII and a 50% of the PI isoenzyme, whereas the levels of glucokinase as well as those of glucose-6-phosphate, fructose-6-phosphate, pyruvate and ATP remained unaffected.The analysis of the enzymes present in cells grown in ethanol, limiting glucose and high glucose, shows that hexokinase PII predominates in cells under catabolic repression, the opposite is true for glucokinase, whereas hexokinase PI remains unaffected.

Protein kinase C from small intestine epithelial cells

Protein kinase C activity has been identified in cytosolic and membrane fractions from rat and rabbit small intestine epithelial cells. The cytosolic fraction comprised about the 75% of total activity. Protein kinase C activity was resolved from other protein kinase activities by ion exchange chromatography. Phosphatidylserine or phosphatidylinositol were required for protein kinase C to be active. In addition, the activity was enhanced by the presence of a diacylglycerol. Diolein and dimyristin were the most effective (13-14 fold activation). In the presence of phosphatidylserine and diolein, the Ka for activation by Ca2+ was 10(-7)M. The phorbol ester TPA substituted for diacylglycerol in activating protein kinase C. Brush border and basolateral membranes contained protein kinase C activity, although the specific activity of the basal lateral membranes was four-fold higher than the specific activity of the brush border membranes. The presence of PKC in small intestine epithelial cells might have important implications in the Ca2+ mediated control of ionic transport in this tissue.