Studies on the properties of cow's-milk tributyrinases and their interaction with milk proteins

Association of lipases with micellar and soluble casein complexes

The association of lipase with casein micelles and soluble casein complexes was investigated by gel-filtration on Sephadex G-200 and Sepharose 2B columns which were equilibrated with synthetic milk serum. Gel-filtration indicated that the molecular weight of casein micelles in milk is > 108whereas the casein in colloidal phosphate-free milk is present as soluble complexes of molecular weightca.2×106containing αs-, β- and κ-casein. The soluble complexes appear to be stabilized in the micelle by colloidal calcium phosphate linkages. On addition of pancreatic lipase to milk, activity was impaired due to binding of the enzyme both to micellar and to soluble casein complexes. The enzyme dissociated from the latter during gel-filtration on Sepharose 2B columns. The binding of lipase to casein was not dependent on the presence of colloidal phosphate and consequently complete micellar structure is not essential for association of lipase with casein. Binding of the lipase to phosvitin did not result in a loss of enzyme activity. Lipases in milk appear to be involved in the equilibrium between micellar and soluble casein. The activity of lipases in milk is apparently influenced by this equilibrium. Some problems encountered in the use of gel-filtration to study the interactions of lipases with caseins are described.

The temperature-dependent dissociation of β-casein from bovine casein micelles and complexes

The temperature-dependent dissociation of β-casein from the casein micelles of milk and from the soluble casein complexes of colloidal phosphate-free (CPF) milk was investigated by high-speed centrifugation and gel-filtration. The percentage of the total casein in supernatants prepared by high-speed centrifugation of mid-lactation milks increased from approximately 6 to 15% on cooling the milks from 30 to 5 °C; β-casein accounted for about 46% of this increase, while αs-and κ-casein constituted 30 and 23%, respectively. On gel-filtration both of skim-milk and CPF milk on Sepharose 2B at 0, 2, 5, 10 and 25 °C, maximum amounts of free β-casein (c. 60% of total) were obtained at 5 °C. The remainder of the β-casein appeared to be more strongly bound to the αs- and κ-casein and may be involved in the internal cohesion of casein micelles. The free β-casein of both milk preparations appeared to be in equilibrium with the bound β-casein. On Sephadex G-200 columns at 5 °C, approximately 5 and 60% of the β-casein of skim-milk and CPF milk, respectively, was eluted in the free form in the expected position for a globular protein of molecular weight about 200000. At low temperatures, particularly at 5 °C, colloidal phosphate appeared to play an integrating role in the association of over half the total β-casein with the other casein components of native micelles. However, when the equilibrium between micellar and free β-casein was disturbed by gel-filtration on Sepharose 2B, the presence of colloidal phosphate did not prevent the release of most of the β-casein from casein micelles. Some problems encountered in the use of densitometry for the estimation of individual caseins on electropherograms are described.

Stability of lipoprotein lipase activity in bovine milk

The effects of temperature, dilution, dialysis and the presence of heparin on the stability of lipoprotein lipase (LPL) in milk, skim-milk, milk serum and casein micelles were investigated. At 4 and 20 °C milk serum was the source of the least stable LPL and casein was that of the most stable. There was little difference between LPL stability in milk and skim-milk at these temperatures, or between serum and casein LPL at 50 °C. Heparin (5 µg/ml) increased stability although the effect was less for casein LPL than for serum LPL. A 40-fold dilution of serum LPL with either simulated milk ultrafiltrate (SMUF) or 0·01 M-Tris-Cl pH 8·3 increased the loss of serum LPL, but not of casein LPL. Dialysis of skim-milk against deionized water or SMUF increased stability at 4 or 20 °C but not at 37 °C. LPL activity was more stable in diluted samples of dialysed skim-milk than in diluted samples of the same milk which had not been dialysed. Dialysis against deionized water increased lipolysis but against SMUF it did not increase. Solutions prepared by dialysing water against some milks were found to inhibit lipolysis and this effect was overcome by heparin. The possibility that milk serum contains a factor which influences LPL stability is discussed.

Physico-chemical characteristics of casein micelles in dilute aqueous media

The heat stability and rennet coagulation time (second stage) of milk were reduced by brief dialysis against water. Destabilization appears to arise from a developed imbalance between Ca and phosphate plus citrate due to the very slow diffusion of Ca on dialysis. Average micelle size as indicated by permeation chromatography in porous glass CPG 10 was slightly reduced by dialysis for 24 h. Direct addition of low levels (10–100 mM) of NaCl to milk markedly reduced heat stability at pH > 7·0 (normal minimum) possibly due to dissociation of κ-casein, but increased rennet coagulation times; higher levels of NaCl decreased heat stability throughout the pH range 6·4–7·4.

Milk lipoprotein lipase distribution in the major fractions of bovine milk

Raw, bovine bulk tank milk and milks from selected cows were separated by ultracentrifugation into four major fractions: casein, sloughed membrane material, serum, and milk fat globule membrane. Milk lipoprotein lipase activity was measured by the pH stat method and protein determinations were made by the Lowry procedure for each of the four fractions in order to calculate specific activity (units per milligram of protein). In six farm-cooled bulk milk samples stored less than or equal to 24 h, casein had a significantly higher milk lipoprotein lipase total activity, 35.66 units/ml of milk, than all of the fractions. Serum had the next highest activity with 11.69 units/ml of milk. Fluff and milk fat globule membrane had activities of .80 and .41 units/ml of milk, respectively. The specific activity of the fluff was 3.3 milk lipoprotein lipase units/mg of protein, which was significantly higher than the casein and serum fractions in pooled milk. Milks from five cows in midlactation were assayed individually for milk lipoprotein lipase activity and protein content immediately after milking and after 12, 24, 48 and 72 h of cold (4 degrees C) storage. Fresh warm milk was characterized by the absence of fluff. Casein had the highest mean activity (29.91 units/ml), followed by serum (10.25 units/ml) and milk fat globule membrane (.26 units/ml) in the warm milk from the individual cows. Upon cooling to 4 degrees C, significant increases in enzyme activity in the fluff and milk fat globule membrane fractions were observed at 12 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipolysis and the distribution of lipase activity in bovine milk in relation to stage of lactation and time of milking

The increase in levels of free fatty acids (FFA) in morning and afternoon milk after 48 h cold storage at 5 degrees C was determined at two stages of lactation in milk from 96 cows. Lipolysis was positively correlated to day of lactation (r = 0.6, P less than 0.001), while the correlations between lipolysis and milk yield and fat content respectively were due to these factors being related to day of lactation. In afternoon milk the level of FFA increased almost three times above that in morning milk. Lipolysis was closely related to the lipoprotein lipase (LPL) activity associated with the cream fraction (r = 0.8, P less than 0.001). Afternoon milk had higher LPL activity in the cream fraction than morning milk. Lipase activity in the cream increased with increasing stage of lactation. Cooling of milk increased LPL activity associated with the fat phase. This effect was greater in afternoon milk from late lactation than in morning milk from early lactation. When heparin was added to milk the LPL activity associated with the fat increased substantially; again, this effect was greater in afternoon milk from late lactation than in morning milk from early lactation. Lipolysis was higher in heparinized milk than in normal milk, and there was a close correlation between lipolysis and LPL activity associated with the fat (r = 0.82, P less than 0.001). Characteristics of the milk fat globule influencing the attraction of LPL were found to be fundamentally important to lipolysis.

Factors affecting the distribution of lipoprotein lipase activity between serum and casein micelles in bovine milk

The influence of mastitis and early lactation, and the effect of treating milk with heparin, blood serum and trypsin, on the proportion of lipoprotein lipase (LPL) activity in mild serum was investigated. The relative importance of milk serum LPL and LPL bound to micellar casein in promoting lipolysis was also examined. Colostrum contained LPL activity, 45% of which was found in the serum phase in samples obtained from the first milking post partum, but this value fell to 34% in samples taken 24 h later. The proportion of serum LPL was also increased in milks from quarters infected with Staphylococcus aureus, but not after overnight treatment of normal milk at 4 degrees C with 5% (w/v) blood serum or 2 microgram/ml trypsin. The addition of 5 microgram/ml heparin resulted in a consistent increase in serum LPL which varied between 14 and 50% of total milk LPL. Heparin did not release all the enzyme bound to casein micelles even after a second heparin treatment of resuspended micelles. Serum LPL was more effective in promoting lipolysis and was more responsive to blood serum activation than LPL bound to casein micelles. Lipolysis increased after heparin treatment but the increase was not related to serum LPL activity.

Enzyme immunoassay for measuring lipoprotein lipase activator in milk

A method for determining the relative amount of lipoprotein lipase activator in milk was developed. The activator was measured in arbitrary units which were based on a standard curve for high density lipoprotein. Activator varied between cows not only in whole milk but also in skim milk and milk serum. Most of the activator was in the skim milk, and the amount of activator in milk serum varied between 32 and 91% of that in skim milk. In seven of eight cows, free fatty acids of milk increased with increasing units of activator.

Observations on the influence of high cell count on lipolysis in bovine milk

The effect that changes in composition which occur in milks possessing high cell counts have on milk lipolysis has been investigated. High cell counts were produced either by intramammary infusion ofEscherichia coliendotoxin orStreptococcus agalactiaeor by addition of washed cells which were isolated from milk obtained from quarters infused with endotoxin. Free fatty acid levels in milk were measured in terms of acid degree value (ADV) either as initial ADV measured immediately after milking or ADV developed after a prescribed incubation period.

There was an increase in initial ADV after the infusion either of endotoxin or ofStr. agalactiaerelative to a control quarter. This increase appeared to be associated with changes in cell count, but in absolute terms the influence of cells on ADV became less as cell count increased. Neither type of infusion had any effect on lipoprotein lipase activity. The addition of washed cells to normal milk resulted in an increase in developed ADV, but the increment was not as large as that produced by the addition of 1% blood serum. When cream and skim-milk from endotoxin-treated quarters and control quarters were mixed in different combinations with and without additional cells, developed ADV was higher in those samples containing endotoxin cream and those with added cells. Milk from a quarter treated with endotoxin was diluted with its own skim-milk to produce different cell counts and ADV was determined after various time intervals at 4 and 37 °C. Lipolysis increased with increasing cell count, but a depression in lipolytic rate was noted after incubation for 6 h at 4 °C and 20 min at 37 °C.

The proportion of skim-milk lipoprotein lipase activity in milk serum was larger both in milks possessing high cell counts and in normal milk adjusted to between 5 and 20 mM-NaCl by addition of solid NaCl. These levels of NaCl inhibited lipolysis.

The possible direct and indirect effects of high cell count on milk lipolysis are discussed.

Lipolytic activity with the membrane fraction of bovine skim milk

Colloidal phosphate-free skim milk was subjected to gel filtration on Sepharose 4B. Lipolytic activity was observed in the membrane material eluted in the void volume fraction and in the protein fraction representing a broad range of molecular weights.

Lipases in bovine milk and the relationship between the lipoprotein lipase and tributyrate hydrolysing activities in cream and skim-milk

The lipoprotein lipase and tributyrate hydrolysing activities were found to be similarly distributed in the fractions obtained when whole milk was separated into skim-milk and cream, and when the cream was washed and freed from lipid. These enzyme activities in skim-milks and in extracts of lipid-free cream could not be separated by affinity chromatography on heparin-Sepharose. The enzymes were inactivated to the same degree when incubated at 37 degrees C in the presence of 1-5 M-NaCl, pH 8-5, and both showed marked decrease in stability at 4 degrees C in UV-light caused the same decrease in both lipoprotein lipase and tributyrate hydrolysing activities. An antiserum against a highly purified skim-milk lipoprotein lipase caused total inhibition of the lipoprotein lipase and tributyrate hydrolysing activities in skim-milk and in extracts of lipid-free cream. It is suggested that in bovine milk there is only one major lipase and that it is identical to lipoprotein lipase.

Evidence for the presence of several lipases n cow's milk

Skim milks containing sodium chloride (0.75m) were centrifuged at 80000g for 2hr. and portions of the supernatants were submitted to gel filtration on columns of Sephadex G-200. Enzymes in the effluent fractions were assayed titrimetrically for their hydrolytic activities towards tributyrin, triolein and milk-fat emulsions, and triacetin solution. Summation of the measurements gave ratios of activities towards the various substrates similar to those of the original skim milks. Although only partial separation was obtained, five enzymes appeared to be present. They showed some differences in substrate specificity, but all appeared to be lipases in that they hydrolysed the emulsified substrates more rapidly than the dissolved triacetin.