Enzymatic iodination of polypeptides with 125I to high specific activity

1. Lactoperoxidase was extracted from cow milk by a simplified method starting with batch-wise adsorption onto GM-Sephadex-50. It was then purified by (NH4)2SO4 precipitation and isoelectric focusing. This product had an A412 nm/A280 nm ratio of 0.8-0.9. 2. Lactoperoxidase together with H2O2 could oxidize carrier-free Na125I to "active iodine" with efficiency to iodinate proteins to high specific activity. 3. Polypeptide hormones radioiodinated by this technique retained their immunological reactivity and were used in radioimmunoassays with good results.

Positional candidate cloning of a QTL in dairy cattle: identification of a missense mutation in the bovine DGAT1 gene with major effect on milk yield and composition

We recently mapped a quantitative trait locus (QTL) with a major effect on milk composition--particularly fat content--to the centromeric end of bovine chromosome 14. We subsequently exploited linkage disequilibrium to refine the map position of this QTL to a 3-cM chromosome interval bounded by microsatellite markers BULGE13 and BULGE09. We herein report the positional candidate cloning of this QTL, involving (1) the construction of a BAC contig spanning the corresponding marker interval, (2) the demonstration that a very strong candidate gene, acylCoA:diacylglycerol acyltransferase (DGAT1), maps to that contig, and (3) the identification of a nonconservative K232A substitution in the DGAT1 gene with a major effect on milk fat content and other milk characteristics.

Crystal structures of bovine milk xanthine dehydrogenase and xanthine oxidase: structure-based mechanism of conversion

Mammalian xanthine oxidoreductases, which catalyze the last two steps in the formation of urate, are synthesized as the dehydrogenase form xanthine dehydrogenase (XDH) but can be readily converted to the oxidase form xanthine oxidase (XO) by oxidation of sulfhydryl residues or by proteolysis. Here, we present the crystal structure of the dimeric (M(r), 290,000) bovine milk XDH at 2.1-A resolution and XO at 2.5-A resolution and describe the major changes that occur on the proteolytic transformation of XDH to the XO form. Each molecule is composed of an N-terminal 20-kDa domain containing two iron sulfur centers, a central 40-kDa flavin adenine dinucleotide domain, and a C-terminal 85-kDa molybdopterin-binding domain with the four redox centers aligned in an almost linear fashion. Cleavage of surface-exposed loops of XDH causes major structural rearrangement of another loop close to the flavin ring (Gln 423Lys 433). This movement partially blocks access of the NAD substrate to the flavin adenine dinucleotide cofactor and changes the electrostatic environment of the active site, reflecting the switch of substrate specificity observed for the two forms of this enzyme.

Proteinases in normal bovine milk and their action on caseins

Native proteolytic enzymes in good quality normal bovine milk readily hydrolysed the caseins during incubation or storage, producing the gamma-caseins, proteose-peptone components 5 (PP5) and 8-fast (PP8F) and a considerable number of other unidentified fragments, many of which were also subsequently found in the proteose-peptone fraction. The rate of casein hydrolysis was greater in pasteurized than in raw milk, with beta-casein being slightly more susceptible to attack than alpha S1-casein. Measurements of gamma-casein and proteose-peptone formation have been made and it was found that PP5 was an intermediate product that was subject to further proteolysis while PP8F was a stable end-product. With the exception of component 3 (PP3), virtually all constituents of the proteose-peptone fraction increased during storage and appeared to be products of the action of proteolytic enzymes. Further evidence was obtained from the effects of various inhibitors that the principal proteinase of normal milk is plasmin, but slight differences were apparent between the protein breakdown patterns induced by storage and by added plasmin, which was consistent with the presence of more than one proteinase. Incubations in the presence of soya bean trypsin inhibitor to prevent plasmin action clearly revealed that another enzyme(s) was also involved.

Secreted kinase phosphorylates extracellular proteins that regulate biomineralization

Protein phosphorylation is a fundamental mechanism regulating nearly every aspect of cellular life. Several secreted proteins are phosphorylated, but the kinases responsible are unknown. We identified a family of atypical protein kinases that localize within the Golgi apparatus and are secreted. Fam20C appears to be the Golgi casein kinase that phosphorylates secretory pathway proteins within S-x-E motifs. Fam20C phosphorylates the caseins and several secreted proteins implicated in biomineralization, including the small integrin-binding ligand, N-linked glycoproteins (SIBLINGs). Consequently, mutations in Fam20C cause an osteosclerotic bone dysplasia in humans known as Raine syndrome. Fam20C is thus a protein kinase dedicated to the phosphorylation of extracellular proteins.

An extremely potent inhibitor of xanthine oxidoreductase. Crystal structure of the enzyme-inhibitor complex and mechanism of inhibition

TEI-6720 (2-(3-cyano-4-isobutoxyphenyl)-4-methyl-5-thiazolecarboxylic acid) is an extremely potent inhibitor of xanthine oxidoreductase. Steady state kinetics measurements exhibit mixed type inhibition with K(i) and K(i)' values of 1.2 +/- 0.05 x 10(-10) m and 9 +/- 0.05 x 10(-10) m, respectively. Fluorescence-monitored titration experiments showed that TEI-6720 bound very tightly to both the active and the inactive desulfo-form of the enzyme. The dissociation constant determined for the desulfo-form was 2 +/- 0.03 x 10(-9) m; for the active form, the corresponding number was too low to allow accurate measurements. The crystal structure of the active sulfo-form of milk xanthine dehydrogenase complexed with TEI-6720 and determined at 2.8-A resolution revealed the inhibitor molecule bound in a long, narrow channel leading to the molybdenum-pterin active site of the enzyme. It filled up most of the channel and the immediate environment of the cofactor, very effectively inhibiting the activity of the enzyme through the prevention of substrate binding. Although the inhibitor did not directly coordinate to the molybdenum ion, numerous hydrogen bonds as well as hydrophobic interactions with the protein matrix were observed, most of which are also used in substrate recognition.

Lactoperoxidase: physico-chemical properties, occurrence, mechanism of action and applications

Lactoperoxidase (LP) is one of the most prominent enzymes in bovine milk and catalyses the inactivation of a wide range of micro-organisms in the lactoperoxidase system (LP-s). LP-systems are also identified as natural antimicrobial systems in human secretions such as saliva, tear-fluid and milk and are found to be harmless to mammalian cells. The detailed molecular structure of LP is identified and the major products generated by the LP-s and their antimicrobial action have been elucidated for the greater part. In this paper several aspects of bovine LP and LP-s are discussed, including physico-chemical properties, occurrence in milk and colostrum and mechanisms of action. Since the introduction of industrial processes for the isolation of LP from milk and whey the interest in this enzyme has increased considerably and attention will be paid to potential and actual applications of LP-systems as biopreservatives in food and other products.

Reduction of nitrite to nitric oxide catalyzed by xanthine oxidoreductase

Xanthine oxidase (XO) was shown to catalyze the reduction of nitrite to nitric oxide (NO), under anaerobic conditions, in the presence of either NADH or xanthine as reducing substrate. NO production was directly demonstrated by ozone chemiluminescence and showed stoichiometry of approximately 2:1 versus NADH depletion. With xanthine as reducing substrate, the kinetics of NO production were complicated by enzyme inactivation, resulting from NO-induced conversion of XO to its relatively inactive desulfo-form. Steady-state kinetic parameters were determined spectrophotometrically for urate production and NADH oxidation catalyzed by XO and xanthine dehydrogenase in the presence of nitrite under anaerobic conditions. pH optima for anaerobic NO production catalyzed by XO in the presence of nitrite were 7.0 for NADH and </=6.0 for xanthine. Involvement of the molybdenum site of XO in nitrite reduction was shown by the fact that alloxanthine inhibits xanthine oxidation competitively with nitrite. Strong preference for Mo=S over Mo=O was shown by the relatively very low NADH-nitrite reductase activity shown by desulfo-enzyme. The FAD site of XO was shown not to influence nitrite reduction in the presence of xanthine, although it was clearly involved when NADH was the reducing substrate. Apparent production of NO decreased with increasing oxygen tensions, consistent with reaction of NO with XO-generated superoxide. It is proposed that XO-derived NO fulfills a bactericidal role in the digestive tract.

The effect of milk and lactobacillus feeding on human intestinal bacterial enzyme activity

Twenty-one subjects were recruited for a study designed to investigate the effect of oral supplements of Lactobacillus acidophilus on fecal bacterial enzyme activity. Three bacterial enzymes were assayed: beta-glucuronidase, nitroreductase, and azoreductase. These fecal enzymes can catalyze procarcinogens conversion to a proximal carcinogen. The sequence of feeding studies and fecal enzyme assays was the same for all subjects: 4 wk of a control period; 4 wk of plain milk feeding; 4 wk of control, without any dietary supplements; 4 wk of milk containing 2 X 10(6) per ml viable L acidophilus; and 4 wk of control, without any supplements. The concentration of viable lactobacilli simulates that found in acidophilus supplemented milk. Reductions of 2- to 4-fold in the activities of the three fecal enzymes were observed only during the period of lactobacilli feeding. These changes were noted in all subjects and were highly significant (p less than 0.02 to 0.001). During the final control period, after lactobacilli feeding, fecal enzyme levels returned to normal after 4 wk.

Long-term intravenous treatment of Pompe disease with recombinant human alpha-glucosidase from milk

OBJECTIVE

Recent reports warn that the worldwide cell culture capacity is insufficient to fulfill the increasing demand for human protein drugs. Production in milk of transgenic animals is an attractive alternative. Kilogram quantities of product per year can be obtained at relatively low costs, even in small animals such as rabbits. We tested the long-term safety and efficacy of recombinant human -glucosidase (rhAGLU) from rabbit milk for the treatment of the lysosomal storage disorder Pompe disease. The disease occurs with an estimated frequency of 1 in 40,000 and is designated as orphan disease. The classic infantile form leads to death at a median age of 6 to 8 months and is diagnosed by absence of alpha-glucosidase activity and presence of fully deleterious mutations in the alpha-glucosidase gene. Cardiac hypertrophy is characteristically present. Loss of muscle strength prevents infants from achieving developmental milestones such as sitting, standing, and walking. Milder forms of the disease are associated with less severe mutations and partial deficiency of alpha-glucosidase.

METHODS

In the beginning of 1999, 4 critically ill patients with infantile Pompe disease (2.5-8 months of age) were enrolled in a single-center open-label study and treated intravenously with rhAGLU in a dose of 15 to 40 mg/kg/week.

RESULTS

Genotypes of patients were consistent with the most severe form of Pompe disease. Additional molecular analysis failed to detect processed forms of alpha-glucosidase (95, 76, and 70 kDa) in 3 of the 4 patients and revealed only a trace amount of the 95-kDa biosynthetic intermediate form in the fourth (patient 1). With the more sensitive detection method, 35S-methionine incorporation, we could detect low-level synthesis of -glucosidase in 3 of the 4 patients (patients 1, 2, and 4) with some posttranslation modification from 110 kDa to 95 kDa in 1 of them (patient 1). One patient (patient 3) remained totally deficient with both detection methods (negative for cross-reactive immunologic material [CRIM negative]). The alpha-glucosidase activity in skeletal muscle and fibroblasts of all 4 patients was below the lower limit of detection (<2% of normal). The rhAGLU was tolerated well by the patients during >3 years of treatment. Anti-rhAGLU immunoglobulin G titers initially increased during the first 20 to 48 weeks of therapy but declined thereafter. There was no consistent difference in antibody formation comparing CRIM-negative with CRIM-positive patients. Muscle alpha-glucosidase activity increased from <2% to 10% to 20% of normal in all patients during the first 12 weeks of treatment with 15 to 20 mg/kg/week. For optimizing the effect, the dose was increased to 40 mg/kg/week. This resulted, 12 weeks later, in normal alpha-glucosidase activity levels, which were maintained until the last measurement in week 72. Importantly, all 4 patients, including the patient without any endogenous alpha-glucosidase (CRIM negative), revealed mature 76- and 70-kDa forms of -glucosidase on Western blot. Conversion of the 110-kDa precursor from milk to mature 76/70-kDa alpha-glucosidase provides evidence that the enzyme is targeted to lysosomes, where this proteolytic processing occurs. At baseline, patients had severe glycogen storage in the quadriceps muscle as revealed by strong periodic acid-Schiff--positive staining and lacework patterns in hematoxylin and eosin--stained tissue sections. The muscle pathology correlated at each time point with severity of signs. Periodic acid-Schiff intensity diminished and number of vacuoles increased during the first 12 weeks of treatment. Twelve weeks after dose elevation, we observed signs of muscle regeneration in 3 of the 4 patients. Obvious improvement of muscular architecture was seen only in the patient who learned to walk. Clinical effects were significant. All patients survived beyond the age of 4 years, whereas untreated patients succumb at a median age of 6 to 8 months. The characteristic cardiac hypertrophy present at start of treatment diminished significantly. The left ventricular mass index decreased from 171 to 599 g/m2 (upper limit of normal 86.6 g/m2 for infants from 0 to 1 year) to 70 to 160 g/m2 during 84 weeks of treatment. In addition, we found a significant change of slope for the diastolic thickness of the left ventricular posterior wall against time at t = 0 for each separate patient. Remarkably, the younger patients (patients 1 and 3) showed no significant respiratory problems during the first 2 years of life. One of the younger patients recovered from a life-threatening bronchiolitis at the age of 1 year without sequelae, despite borderline oxygen saturations at inclusion. At the age of 2, however, she became ventilator dependent after surgical removal of an infected Port-A-Cath. She died at the age of 4 years and 3 months suddenly after a short period of intractable fever of >42 degrees C, unstable blood pressure, and coma. The respiratory course of patient 1 remained uneventful. The 2 older patients, who both were hypercapnic (partial pressure of carbon dioxide: 10.6 and 9.8 kPa; normal range: 4.5-6.8 kPa) at start of treatment, became ventilator dependent before the first infusion (patient 2) and after 10 weeks of therapy (patient 4). Patient 4 was gradually weaned from the ventilator after 1 year of high-dose treatment and was eventually completely ventilator-free for 5 days, but this situation could not be maintained. Currently, both patients are completely ventilator dependent. The most remarkable progress in motor function was seen in the younger patients (patients 1 and 3). They achieved motor milestones that are unmet in infantile Pompe disease. Patient 1 learned to crawl (12 months), walk (16 months), squat (18 months), and climb stairs (22 months), and patient 3 learned to sit unsupported. The Alberta Infant Motor Scale score for patients 2, 3, and 4 remained far below p5. Patient 1 followed the p5 of normal.

CONCLUSION

Our study shows that a safe and effective medicine can be produced in the milk of mammals and encourages additional development of enzyme replacement therapy for the several forms of Pompe disease. Restoration of skeletal muscle function and prevention of pulmonary insufficiency require dosing in the range of 20 to 40 mg/kg/week. The effect depends on residual muscle function at the start of treatment. Early start of treatment is required.

Indicators of inflammation in the diagnosis of mastitis

Mastitis affects the quality of milk and is a potential health risk for the other cows. In a well managed dairy herd, in addition to clinical mastitis, subclinical mastitis should be efficiently detected. Bacteriological sampling is not feasible as a routine test to identify subclinical mastitis, and indirect tests of mastitis are more suitable for selecting cows with intramammary infections for subsequent bacteriological sampling. Mastitis affects the composition of milk, and the degree of changes depends on the infecting agent and the inflammatory response. Indicators of inflammation in the milk which can be determined using rapid, reliable and easy routine techniques, can be used for the early detection of mastitis. The measuring of the somatic cell count in milk is the standard method, but the analysis technique is problematic for routine use in herds. The most promising parameters for monitoring subclinical mastitis are milk N-acetyl-beta-D-glucosaminidase activity, lactose, and electrical conductivity along with some other indicators such as optical and milk flow measurements, preferably with an inter-quarter evaluation included in the test. Acute phase proteins, haptoglobin and serum amyloid A, are also potential candidates for mastitis monitoring. New mastitis detection systems which can be adapted into on-line use are urgently needed, since dairy units are growing bigger and automatic milking systems are being taken into use.

Selectivity of febuxostat, a novel non-purine inhibitor of xanthine oxidase/xanthine dehydrogenase

The purine analogue, allopurinol, has been in clinical use for more than 30 years as an inhibitor of xanthine oxidase (XO) in the treatment of hyperuricemia and gout. As consequences of structural similarities to purine compounds, however, allopurinol, its major active product, oxypurinol, and their respective metabolites inhibit other enzymes involved in purine and pyrimidine metabolism. Febuxostat (TEI-6720, TMX-67) is a potent, non-purine inhibitor of XO, currently under clinical evaluation for the treatment of hyperuricemia and gout. In this study, we investigated the effects of febuxostat on several enzymes in purine and pyrimidine metabolism and characterized the mechanism of febuxostat inhibition of XO activity. Febuxostat displayed potent mixed-type inhibition of the activity of purified bovine milk XO, with Ki and Ki' values of 0.6 and 3.1 nM respectively, indicating inhibition of both the oxidized and reduced forms of XO. In contrast, at concentrations up to 100 muM, febuxostat had no significant effects on the activities of the following enzymes of purine and pyrimidine metabolism: guanine deaminase, hypoxanthine-guanine phosphoribosyltransferase, purine nucleoside phosphorylase, orotate phosphoribosyltransferase and orotidine-5'-monophosphate decarboxylase. These results demonstrate that febuxostat is a potent non-purine, selective inhibitor of XO, and could be useful for the treatment of hyperuricemia and gout.

The crystal structure of xanthine oxidoreductase during catalysis: implications for reaction mechanism and enzyme inhibition

Molybdenum is widely distributed in biology and is usually found as a mononuclear metal center in the active sites of many enzymes catalyzing oxygen atom transfer. The molybdenum hydroxylases are distinct from other biological systems catalyzing hydroxylation reactions in that the oxygen atom incorporated into the product is derived from water rather than molecular oxygen. Here, we present the crystal structure of the key intermediate in the hydroxylation reaction of xanthine oxidoreductase with a slow substrate, in which the carbon-oxygen bond of the product is formed, yet the product remains complexed to the molybdenum. This intermediate displays a stable broad charge-transfer band at approximately 640 nm. The crystal structure of the complex indicates that the catalytically labile Mo-OH oxygen has formed a bond with a carbon atom of the substrate. In addition, the MoS group of the oxidized enzyme has become protonated to afford Mo-SH on reduction of the molybdenum center. In contrast to previous assignments, we find this last ligand at an equatorial position in the square-pyramidal metal coordination sphere, not the apical position. A water molecule usually seen in the active site of the enzyme is absent in the present structure, which probably accounts for the stability of this intermediate toward ligand displacement by hydroxide.

A sensitive fluorometric assay for measuring xanthine dehydrogenase and oxidase in tissues

The conversion of xanthine dehydrogenase to a free radical producing oxidase is an important component of oxygen-mediated tissue injury. Current assays for these enzymes are of limited sensitivity, making it difficult to analyze activities in organ biopsies or cultured cells. The xanthine oxidase-catalyzed conversion of pterin (2-amino-4-hydroxypteridine) to isoxanthopterin provides the basis for a fluorometric assay which is 100-500 times more sensitive than the traditional spectrophotometric assay of urate formation from xanthine. Enzyme activity as low as 0.1 pmol min-1 ml-1 can be measured with the fluorometric pterin assay. Xanthine oxidase is assayed in the presence of pterin only, while combined xanthine dehydrogenase plus oxidase activity is determined with methylene blue which replaces NAD+ as an electron acceptor. The relative proportions and specific activities of xanthine oxidase and dehydrogenase determined by the fluorometric pterin assay are comparable with the spectrophotometric measurement of activities present in rat liver, intestine, kidney, and plasma. The assay has been successfully applied to brain, human kidney, and cultured mammalian cells, where xanthine dehydrogenase and oxidase activities are too low to detect spectrophotometrically.