Characterization of polyphenol oxidase in coffee

Polyphenol oxidase (PPO) was characterized in partially purified extracts of leaves (PPO-L) and fruit endosperm (PPO-E) of coffee (Coffea arabica L.). PPO activity was higher in early developmental stages of both leaves and endosperm of fruits. Wounding or exposure of coffee leaves to methyl jasmonate increased PPO activity 1.5-4-fold. PPO was not latent and was not activated by protease treatment. PPO activity was stimulated 10-15% with sodium dodecyl sulphate (SDS) at 0.35-1.75 mM, but at higher concentrations activities were similar to the control samples, without detergent. Prolonged incubation of extracts with trypsin or proteinase K inhibited PPO activity but pepsin had no effect. Inhibition of PPO with proteinase K was increased in the presence of SDS. PPO activity from both tissues was optimal at pH 6-7 and at an assay temperature of 30 degrees C. Activity was highest with chlorogenic acid as substrate with a Km of 0.882 mM (PPO-L) and 2.27 mM (PPO-E). Hexadecyl trimethyl-ammonium bromide, polyvinylpyrrolidone 40. cinnamic acid and salicylhydroxamic acid inhibited PPO from both tissues. Both enzymes were inactivated by heat but the activity in endosperm extracts was more heat labile than that from leaves. The apparent Mr determined by gel filtration was 46 (PPO-L) and 50 kDa (PPO-E). Activity-stained SDS polyacrylamide gel electrophoresis (PAGE) gels and western blots probed with PPO antibodies suggested the existence of a 67 kDa PPO which is susceptible to proteolytic cleavage that generates a 45 kDa active form.

Hydrolytic activity of alpha-galactosidases against deoxy derivatives of p-nitrophenyl alpha-D-galactopyranoside

The four possible monodeoxy derivatives of p-nitrophenyl (PNP) alpha-D-galactopyranoside were synthesized, and hydrolytic activities of the alpha-galactosidase of green coffee bean, Mortierella vinacea and Aspergillus niger against them were elucidated. The 2- and 6-deoxy substrates were hydrolyzed by the enzymes from green coffee bean and M. vinacea, while they scarcely acted on the 3- and 4-deoxy compounds. On the other hand, A. niger alpha-galactosidase hydrolyzed only the 2-deoxy compound in these deoxy substrates, and the activity was very high. These results indicate that the presence of two hydroxyl groups (OH-3 and -4) is essential for the compounds to act as substrates for the enzymes of green coffee bean and M. vinacea, while the three hydroxyl groups (OH-3, -4, and -6) are necessary for the activity of the A. niger enzyme. The kinetic parameters (K(m) and Vmax) of the enzymes for the hydrolysis of PNP alpha-D-galactopyranoside and its deoxy derivatives were obtained from kinetic studies.

The carboxyl terminus of coffee bean alpha-galactosidase is critical for enzyme activity

The role of the carboxyl (C)-terminal region of coffee bean alpha-galactosidase (alpha-GAL) has been studied by expressing C-terminal deletion mutants in the methylotrophic yeast strain Pichia pastoris. A previous study of human alpha-galactosidase determined that enzyme activity increased when up to 10 amino acid residues were deleted. Deleting 11 residues reduced activity, and deleting 12 residues abolished activity. In our studies, alpha-GAL activity is reduced when one or two amino acids are deleted, as is enzyme secretion directed by P. pastoris signal sequences. The pH profile is similar to that of the wild-type enzyme. Deleting 3 or more residues from the C-terminal end results in a complete loss of both enzyme secretion and activity. The C-terminus of alpha-GAL seems to play an important role in overall enzyme conformation and may directly affect the proper conformation of the active site.

Pig xenogeneic antigen modification with green coffee bean alpha-galactosidase

Green coffee bean alpha-galactosidase can cleave the terminal alpha-galactose (alphaGal) on oligosaccharides that form the major antigen on pig endothelial cells recognized by primate-specific antibodies. Studies have been made of the conditions under which it is functional (e.g. temperature, pH) and of its biochemical and immunologic effects. Pig-to-rhesus monkey vein transplants were studied to identify the efficiency of the enzyme in delaying hyperacute rejection. When a graft became occluded, biopsies were taken for light microscopy (hematoxylin and eosin), scanning electron microscopy (SEM) and immunostaining with Griffonia simplicifolia IB4 lectin (GSIB4), and for IgM, IgG and C3. alpha-Galactosidase was stable for 72-96 h and was effective at 4 degrees C and pH 6.9 (conditions of human liver graft storage), although better function was obtained at 20 degrees C and pH 6.5. Using the porcine PK15 cell assay, the cytotoxicity of human serum was reduced after treatment of the pig cells with the enzyme. In vitro studies demonstrated that porcine veins treated with alpha-galactosidase lost endothelial expression of the Gal epitope within 30 min. SEM, however, demonstrated endothelial damage beginning within 2 h, probably caused by the alpha-galactosidase, as no damage was found in phosphate-buffered saline-treated veins, where the Gal epitope was preserved for >3 h. No change was found in either group on light microscopy. In vivo studies demonstrated that patency of the alpha-galactosidase-treated veins (mean 2.5 h) was longer than that of untreated veins (0.23 h) (P < 0.01). Biopsies showed no GSIB4 lectin staining for alpha-Gal epitopes and much less IgM and C3 deposition in the treated group. Light microscopy and SEM demonstrated more severe endothelial damage, hemorrhage, and fibrin formation in the untreated group. Galactosidase is effective in removing the terminal alphaGal and delays the onset of hyperacute rejection of pig veins transplanted into monkeys. However, its effect is temporary and, on its own, its use is unlikely to prolong survival of pig organs transplanted into primates sufficiently to be of clinical value.

Enzymatic transformations in supersaturated substrate solutions: II. Synthesis of disaccharides via transglycosylation

Enzymatic transglycosylation in supersaturated solutions of substrates was investigated using crude glycosidase preparations from barley, snail, and coffee beans. It was shown that the use of supersaturated glycoside solutions as media for transglycosylation reactions offers considerable advantages over conventional aqueous systems. These advantages include higher yields, more efficient use of the donor glycosides and improved volumetric productivity, especially in the case of poorly water-soluble substrates. The regioselectivity of the glycosylation was not significantly affected by high concentrations of acceptor glycosides. It was also shown that the regioselectivity of transfer could be directed towards secondary hydroxyl groups by the use of methyl 6-O-acetyl-alpha-galactopyranoside as acceptor. The value of these approaches was demonstrated by the synthesis of methyl 3- and 4-O-beta-D-galactopyranosyl-alpha-D-galactopyranosides and methyl 3-O-beta-D-galactopyranosyl-alpha-L-fucopyranoside on a preparative scale.

Enzymatic synthesis, isolation, and analysis of novel alpha- and beta-galactosyl-cycloisomalto-octaoses

Novel branched cycloisomalto-octaoses (CI8s) were enzymatically synthesized by transgalactosylation with alpha-galactosidase from coffee bean and beta-galactosidase preparations from Penicillium multicolor and Bacillus circulans, using melibiose and lactose as donor substrates, and CI8 which is a cyclic homogeneous oligosaccharide composed of eight glucose units bound by alpha-(1-->6)-linkages, as an acceptor. alpha-Galactosyl-CI8s and beta-galactosyl-CI8s obtained were isolated and purified by HPLC. Their structures were elucidated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOFMS) and NMR spectroscopy.

Separation of the N-7 methyltransferase, the key enzyme in caffeine biosynthesis

Caffeine biosynthesis comprises sequential methylations at N-7, N-3 and N-1 of the xanthine ring catalysed by S-adenosyl-L-methionine (SAM)-dependent methyltransferase activities that, to date, have not been resolved. Enzyme extracts were prepared from young, emerging coffee leaflets and following anion exchange chromatography, chromatofocusing facilitated the clear separation of the N-7-methyltransferase from the N-3- and N-1-methyltransferase activities. All three N-methyltransferases co-eluted when analysed by gel filtration chromatography and their native molecular mass was ca 67 kDa. Photoaffinity labelling with [methyl-3H]SAM followed by SDS-PAGE of a chromatofocusing-purified preparation containing only N-7-methyltransferase activity demonstrated the presence of a single labelled band of 40 kDa. Similar analysis of a gel filtration purified preparation containing all three N-methyltransferase activities revealed the presence of three labelled bands at 49, 43 and 40 kDa. It remains to be determined whether the 49 and 43 kDa bands are associated with the N-3 and N-1-methyltransferases or whether they are unrelated SAM-dependent methyltransferases or other SAM-binding proteins.

Trp-16 is essential for the activity of alpha-galactosidase and alpha-N-acetylgalactosaminidase

By expressing site-directed mutants in the methylotrophic yeast strain Pichia pastoris, the role of a tryptophan residue at position 16 in the activity of alpha-galactosidase and alpha-N-acetylgalactosaminidase, two closely related exoglycosidases, was studied. A substitution of Trp-16 with an arginine residue in alpha-N-acetylgalactosaminidase abolished the enzyme activity, which was confirmed by replacing a 600 bp fragment containing the mutation with the corresponding wild-type sequence. The same tryptophan residue was then substituted with an alanine in both enzymes by site-directed mutagenesis to reveal a possible relationship between their active sites. The purified alpha-N-acetylgalactosaminidase mutant demonstrated a specific activity of 2.8 x 10(-2) U/mg and a Vmax/K(m) of 4.3 x 10(-2), which were both more than a thousandfold lower than corresponding values for the wild-type enzyme. Furthermore, the mutant failed to bind to an affinity resin, suggesting the involvement of Trp-16 in substrate-binding. In addition, the purified alpha-galactosidase mutant resulted in more than a 10(4)-fold decrease in specific activity. Thus our data suggest that Trp-16 in both alpha-galactosidase and alpha-N-acetylgalactosaminidase is critical for enzymatic activity, which in turn supports the hypothesis that these two enzymes may share a catalytic mechanism involving similar residues in their active sites.

Characterization of coffea canephora alpha-D-galactosidase blood group B activity

Enzymatic conversion of type B to O erythrocytes with Coffea (coffee bean) alpha-D-galactosidase was first described by Harpaz and Flowers and subsequently adopted by others (1,2). An enzyme-linked immunosorbent assay (ELISA) and soluble oligosaccharide substrates were used to study deantigenation of B erythrocytes with the Coffea enzyme. For the ELISA, microtiter wells were coated with B membranes and treated with enzyme under a variety of conditions, probed with primary IgM monoclonal anti-B followed by secondary anti-murine mu-chain specific alkaline phosphatase conjugate, then developed with substrate. This technique has allowed the rapid determination of enzymatic activity over a broad range of conditions; the purpose being to determine parameters for efficiently enhancing enzyme activity. Solid phase activity was then compared to activity against soluble oligosaccharide substrates. We have determined that, under the conditions tested, only moderate increases in enzyme activity against the B epitope can be achieved by modifying reaction conditions with the native Coffea enzyme.

Characterization of recombinant alpha-galactosidase for use in seroconversion from blood group B to O of human erythrocytes

Alpha-Galactosidase (alpha-GAL) purified from green coffee bean cleaves the terminal galactose residues from the surface of group B erythrocytes, thereby converting these cells serologically to group O cells. Such enzymatically converted red cells have been transfused into group A and O recipients as part of the first phase of FDA-approved clinical trials. Recently we expressed the recombinant alpha-GAL (r)alpha-GAL) in large quantities in a methylotrophic yeast strain Pichia pastoris and purified the protein to apparent homogeneity by chromatography on a macro prep S50 column. Purified (r)alpha-GAL, migrating as a single band of 41 kDa on a SDS-PAGE, appears to be identical to its native counterpart in specific activity (32 U/mg) and kinetic parameters (K(m) =0.363 mM and V(max) = 46.9 U/mg). Both enzymes demonstrate the same pH profile in the pH range from 2 to 9, with an optimal pH at 6.4 when tested with the substrate p-nitrophenol-alpha-D-galactopyranoside. Furthermore, as with its native counterpart, (r)alpha-GAL specifically cleaves alpha-linked terminal galactose residues from group B red cells without affecting other major antigens on the red cell surface. In addition, we developed a method for using RT-PCR to detect possible DNA contamination in the purified protein preparation, which is one of the concerns for in vivo studies. Thus, with a simple procedure for over-expression and purification of (r)alpha-GAL from P. pastoris culture, one can readily obtain the enzyme needed for large-scale sero-conversion of red cells.

Expression and characterization of recombinant alpha-galactosidase in baculovirus-infected insect cells

A cDNA encoding coffee bean alpha-galactosidase was subcloned into baculovirus expression vectors, pVL-1393 and pAc-GP67B, for intracellular and extracellular expression in Spodoptera frugiperda (Sf9) insect cells, respectively. The expressed protein (recombinant alpha-galactosidase) was immunologically reactive with antisera raised against its native counterpart isolated from coffee beans and was biologically active towards the substrate p-nitrophenyl alpha-galactopyranoside. The subcellular distribution of recombinant alpha-galactosidase expressed from different vectors was analyzed by Western blotting, immunofluorescent labeling, and electron microscopy. In addition, recombinant alpha-galactosidase was compared to the native enzyme with respect to glycosylation, thermostability, and pH profile. Furthermore, a recombinant alpha-galactosidase molecule with a His6 tag at its C-terminus was constructed by an overlap PCR method so that the enzyme expressed in Sf9 cells can be purified by a simple affinity chromatography procedure.

Exoglycosidase purity and linkage specificity: assessment using oligosaccharide substrates and high-pH anion-exchange chromatography with pulsed amperometric detection

Simplified HPLC protocols to determine the activity and linkage specificity and to detect the most commonly-encountered contaminants in available exoglycosidase preparations (Jacob and Scudder, Methods Enzymol., 230, 280-300, 1994) were developed. Monosaccharides and oligosaccharides were analyzed in a single chromatographic step using high-pH anion-exchange chromatography with pulsed amperometric detection. All analyses were performed with underivatized oligosaccharide substrates and by direct injection of unprocessed, diluted enzyme digests into the chromatograph. The sialidase from Newcastle disease virus was found to release both alpha (2-->3)- and alpha (2-->6)-linked Neu5Ac from a triantennary, lactosamine-type oligosaccharide. The activity of alpha-galactosidase from green coffee beans was assayed using Gal alpha(1-->3)[Fuc-alpha(1ar2)]Gal by detection of Gal and Fuc alpha(1-->3)Gal. The linkage specificities of beta-galactosidases from Streptococcus pneumoniae and bovine testis were assessed using Gal beta(1-->3 or 4)GlcNAc beta(1-->3)beta(1-->4)Glc as substrates. Contaminating beta-N-acetylhexosaminidase activity in the beta-galactosidase preparation was assayed using an agalactobiantennary oligosaccharide. The alpha(1-->3 or 4) linkage specificity of fucosidase III from almond meal was confirmed (Scudder et al., J. Biol. Chem. 265, 16472-16477, 1990) by its inactivity against a biantennary oligosaccharide with all Fuc residues linked alpha(1-->6). An alpha-fucosidase from chicken liver was found to cleave alpha(1-->2,3 or 6)-linked Fuc residues from oligosaccharides. The activity of jack bean (Canavalia ensiformis) alpha-mannosidase was assayed with a relatively resistant substrate, Man alpha(1-->3)- Man beta(1-->4)GlcNAc. A GlcNAc beta(1-->4)-terminated triantennary oligosaccharide was used to assay for contaminating beta-N-acetylhexosaminidase activity in alpha-mannosidase preparations and to determine the linkage and branch specificity of beta-N-acetylhexosaminidase at different enzyme concentrations.

High-level expression and purification of coffee bean alpha-galactosidase produced in the yeast Pichia pastoris

alpha-Galactosidase isolated from coffee beans cleaves the terminal alpha-galactose residues from oligosaccharide chains on blood group B red cells, thus generating group O cells. Such enzymatically converted red cells not only maintain full erythrocyte integrity and viability in vitro, but also demonstrate immune tolerance and a normal life span in vivo. In order to produce large quantities of recombinant alpha-galactosidase for use in the study of blood-type conversion, we subcloned the cDNA coding for coffee bean alpha-galactosidase into the EcoRI site of the vector pPIC9 in order to express the enzyme in Pichia pastoris, a methylotrophic yeast strain. After P. pastoris transformation, colonies were screened for high-level expression of alpha-galactosidase, based on enzyme activity. In order to increase enzyme production, the growth conditions in the shake flask culture and fermentor culture were optimized. Under the conditions applied, biologically active alpha-galactosidase was produced and secreted into the culture medium at a level of approximately 0.4 g per liter of the fermentor culture. The protein was purified to apparent homogeneity by a simple chromatography procedure, as suggested by a single band of 41 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Its homogeneity was further confirmed by chromatofocusing and N-terminal sequencing. P. pastoris appears to be the choice as host for the large-scale production of recombinant alpha-galactosidase used for blood type conversion.

Identification of tyrosine 108 in coffee bean alpha-galactosidase as an essential residue for the enzyme activity

The cDNA for coffee bean alpha-galactosidase (alpha-Gal) has been cloned and expressed in a baculovirus expression system. An early study of coconut alpha-Gal by chemical modification suggested that one tyrosine residue is at or near the active site. In order to identify such a critical residue, we replaced two tyrosine residues (positions 108 and 158) with phenylalanine by site-directed mutagenesis. The mutated DNA strands, as well as the wild-type ones, were subcloned into pVL vector and transformed into Sf9 insect cells for intracellular expression. The replacement of Tyr-158 with phenylalanine resulted in a mutant alpha-Gal (Y158F) which retained approx. 88% of the activity of wild-type enzyme. However, the substitution of Tyr-108 by phenylalanine (Y108F) almost abolished the enzymatic activity (1.8% of wild-type activity). The Vmax/Km value for the mutant Y108F was 0.027, which was over a 1000-fold lower than that of wild-type alpha-Gal. Our data suggest that Tyr-108 is critical for the enzymatic activity of alpha-Gal.

Galactosylation of cyclodextrins and branched cyclodextrins by alpha-galactosidases

Transgalactosylated derivatives of cyclodextrins (CDs) and glucosyl and maltosyl CDs (G1- and G2-CDs) were synthesized by alpha-galactosidases from coffee bean and Mortierella vinacea (M. vinacea). The structures of the transfer products were analyzed by FAB-mass, 13C-NMR and methylation. Coffee bean alpha-galactosidase transferred a galactosyl residue not only to side chains of G1-CDs and G2-CDs, but also directly to CD rings. M. vinacea alpha-galactosidase transferred a galactosyl residue only to side chains of G2-CDs.

Cloning and functional expression of a cDNA encoding coffee bean alpha-galactosidase

Purified coffee bean alpha-galactosidase (alpha Gal) has been used for removing terminal alpha-galactose residues from the glyco-conjugates at the red cell surface, in studies of blood group conversion. Here, we report the isolation and sequence of the full-length clone for coffee bean alpha Gal by using the polymerase chain reaction (PCR) and rapid amplification of cDNA ends (RACE) techniques. The cDNA clone (1.4 kb) contains a single open reading frame which encodes a protein of 378 amino acids (aa). Its authenticity is confirmed by perfect alignment of aa sequences obtained from purified coffee bean alpha Gal, and by immune reaction with the antibody raised against the enzyme. Furthermore, the protein produced in insect cells shows enzymatic activity towards a synthetic alpha Gal substrate, p-nitro-phenyl-alpha-galactopyranoside.

Stereochemistry of D-galactal and D-galacto-octenitol hydration by coffee bean alpha-galactosidase: insight into catalytic functioning of the enzyme

Green coffee bean alpha-galactosidase was found to catalyze the hydration of D-galactal and (Z)-3,7-anhydro-1,2-dideoxy-D-galacto-oct-2-enitol (D-galacto-octenitol), each a known substrate for beta-galactosidase. The hydration of D-galactal by the alpha-galactosidase in D2O yielded 2-deoxy-2(S)-D-[2-2H]galactose; the hydration of D-[2-2H]galacto-octenitol in H2O yielded 1,2-dideoxy-2(R)-D-[2-2H]galactooct-3-ulose. Thus, the enzyme protonated each substrate from beneath the plane of the ring, as assumed for alpha-D-galactosides. These results provide an unequivocal assignment of the orientation of an acidic catalytic group to the alpha-galactosidase reaction center. In addition, they reveal a pattern of glycal/exocyclic enitol/glycoside protonation by the enzyme that differs from the pattern reported for beta-galactosidase and from that reported for alpha-glucosidases. Further findings show that D-galacto-octenitol is hydrated by the coffee bean alpha-galactosidase to form the alpha-anomer of 1,2-dideoxy-D-galactooctulose and by Escherichia coli beta-galactosidase to form the beta-anomer. That each enzyme converts this enolic substrate to a product whose de novo anomeric configuration matches that formed from its D-galactosidic substrates provides new evidence for the role of protein structure in controlling the steric outcome of reactions catalyzed by these and other glycosylases. The findings are discussed in light of the concept that catalysis by glycosidases involves a "plastic" protonation phase and a "conserved" product configuration phase.

Purification and characterization of a Coffea canephora alpha-D-galactosidase isozyme

Exoglycosidases modify carbohydrate epitopes on glycoproteins and glycolipids. The alpha-D-galactosidase from Coffea canephora is an important exoglycosidase which degrades the human blood group B epitope. Although multiple isozymes have been described, they have never been demonstrably purified and thoroughly characterized. We have developed a technique to purify an isozyme to homogeneity. The isolated enzyme has a molecular weight of 36.7 kDa by SDS PAGE and 34.0 kDa by gel filtration. The isozyme is highly selective for alpha-D-galactosides and inactive against other low molecular weight substrates. It hydrolyzes the the terminal alpha-D-galactosyl residue from the blood group B epitope. Protease activity is below detectable limits. The isozyme has a broad pH optima at 6.3, a pl of 7.03, is unaffected by ionic strength, and is stable at 4 degrees C.

Endocytotic internalization of alpha-2-macroglobulin: alpha-galactosidase conjugate by cultured fibroblasts derived from Fabry hemizygote

Endocytotic internalization of alpha-galactosidase by cultured fibroblasts derived from a patient with Fabry's disease was achieved via receptor-mediated endocytosis of alpha-2-macroglobulin (alpha-2-M). alpha-galactosidase of coffee beans was conjugated to alpha-2-M when the latter was treated with trypsin. Internalization of the conjugate resulted in an increase of alpha-galactosidase activity in the crude cell extracts. The observed internalization was blocked by the presence of bacitracin, an inhibitor of binding between alpha-2-M and its receptor on the cell surface. When the cells were incubated at 4 degrees C with the conjugate, internalization was also inhibited. The alpha-galactosidase activity in the cells was saturated when the concentration of the conjugate in the medium was 40 micrograms/ml. Since non-conjugated alpha-galactosidase was not effectively internalized, the observed internalization of the conjugate was mediated by recognition of alpha-2-M by its receptor. The effective internalization of alpha-galactosidase described in this paper has a potential use in the enzyme replacement therapy of Fabry's disease.

Headspace gas chromatographic determination of beta-galactosidase activity using electron-capture detection

A headspace gas chromatographic method for the determination of beta-galactosidase (E.C. 3.2.1.23) activity is described. The method, in which 2,2,2-trichloroethyl beta-D-galactopyranoside (beta-TCG) is used as substrate, involves determination of the liberated 2,2,2-trichloroethanol by gas chromatography with electron capture detection. The preparation of beta-TCG and of 2,2,2-trichloroethyl alpha-D-galactopyranoside is described. A Km = 0.80 mM was found for the enzymatic hydrolysis of beta-TCG employing beta-galactosidase from Escherichia coli. The assay has been applied to the quantitative determination of E. coli bacteria.

Action of alpha-galactosidase from Clostridium sporogenes and coffee beans on blood group B antigen of erythrocytes. The effect on the viability of erythrocytes in circulation

The effect of alpha-galactosidase, purified from Clostridium sporogenes (Maebashi), was examined on erythrocytes from rats, rabbits and gibbons. The amount of galactose released by alpha-galactosidase from Cl. sporogenes and from coffee beans was compared. The amount of sialic acid released by Vibrio cholera sialidase was also determined. Loss of blood group B specificity following treatment with alpha-galactosidase was demonstrated with anti-B lectin. In animal models, removal of all the alpha-galactosyl residues with the coffee bean or clostridial alpha-galactosidase resulted in no change in the sequestration pattern of the treated erythrocytes over a period of several days. In contrast, erythrocytes treated with sialidase were rapidly sequestered from the circulation.

Purification and properties of coffee-bean alpha-D-galactosidase

A purification method for alpha-D-galactosidase from Coffea canephora is described. Two enzymes, alpha-D-galactosidases I and II, having molecular weights of 28,000 and 36,500, respectively, were found and extensively purified. The reaction mechanism of alpha-D-galactosidase II was studied. The enzyme hydrolyzed aryl and alkyl alpha-D-galactopyranosides and was severely inhibited by excess of these substrates. No inhibition occurred with raffinose. The influence of para substituents on the reaction rate of phenyl alpha-D-galactopyranosides, the effect of added alcohols, and the non-competitive inhibition by methyl alpha-D-galactopyranoside were investigated. A two-step mechanism with the formation of an enzyme-galactosyl complex is proposed. With aryl galactopyranosides, the reaction of the enzyme-galactosyl complex with water is rate-limiting. Influences of the substituents on the inhibition constant were investigated by linear free-energy relationships, and significant correlations between this constant and electronic parameters could be calculated. The influence of pH on the reaction is complex.