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

Effects of feeding rumen-degradable valine on milk production in late-lactating dairy cows

The study objective was to determine if feeding the rumen-degradable AA Val can increase milk production comparable to recombinant bovine somatotropin (bST). Eight multiparous late-lactating (255±26.4 d in milk) Holstein dairy cows were blocked by milk yield (34.1±8.25 kg/d) and randomly assigned to 1 of 4 treatments in a replicated 4×4 Latin square design with 21-d periods (7 d for dietary adaptation and 14 d for data collection). Treatments were control (CON), a single injection of recombinant bST (rbST), and Val fed at 40 (V40) and 80 g/d (V80). Cows were fed a total mixed ration with a distillers dried grains carrier at 113.4 g/d containing none or added AA. Dry matter intake (21.3, 22.0, 22.8, and 21.5 kg/d for CON, rbST, V40, and V80, respectively) was similar among treatments, except cows receiving V40 had greater dry matter intake than cows receiving V80. Milk yield (22.0, 26.1, 25.2, and 24.9 kg/d), 3.5% fat-corrected milk (22.1, 25.4, 24.4, and 24.3 kg/d), and energy-corrected milk (22.7, 26.1, 25.1, and 24.9 kg/d) were increased at similar amounts for cows receiving rbST, V40, and V80 compared with CON cows. Milk fat percentages (3.51, 3.36, 3.32, and 3.38%) were greatest for CON cows compared with cows receiving V40, whereas cows receiving other treatments were intermediate and similar. Milk protein percentages (3.20, 3.12, 3.15, and 3.13%) were greater for CON cows compared with cows receiving rbST and V40, whereas cows receiving V80 were intermediate and similar. Ruminal isobutyrate (1.19, 1.24, 1.44, and 1.74 mol/100 mol) concentrations were increased for cows receiving V40 and V80 compared with CON and rbST cows, with cows receiving V80 having greater concentrations than cows receiving V40. Plasma growth hormone concentrations (1.78, 1.99, 1.55, and 1.45 ng/mL) were greater for cows receiving rbST compared with cows receiving V40 and V80, whereas CON cows were intermediate and similar. Plasma insulin-like growth factor-1 concentrations (60.4, 106.1, 65.9, and 58.3 ng/mL) were greater for cows receiving rbST compared with cows receiving other treatments. This study suggests that feeding rumen degradable Val can increase milk yield comparable to recombinant bST.

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.

Influence of processing temperature and seasonal change in diet on lipase activity and lipolysis during the mechanical separation of bovine milk

The effect of separating conditions on lipase activity and free fatty acid levels in preheated milk, cream and skim milk was measured on nine occasions during a 12-week period covering the seasonal change from winter feeding to summertime grazing. This change consisted of four periods each representing a different type of forage intake, namely: silage, kale, daytime grazing and 24 h grazing. Milk was separated at 30, 40, 50 and 60°C with preheating times of 10, 25 and 55 s. Results were expressed both as absolute values and in terms of changes relative to the original unheated milk. Lipase activity and free fatty acid concentration were significantly reduced as separation temperature increased but were not influenced by holding time. The loss of activity in cream was progressive so that at 60°C only 40% of the original activity remained. Up to 50°C little change occurred in preheated milk or skim milk activity, while at 60°C 83 and 76% respectively of the original activity remained. The amount of activity calculated to be associated with the fat fraction of the cream also decreased with temperature. Activity varied significantly with date; maximum values were observed during the first 3 weeks of summertime grazing. Relative activity values indicated that the susceptibility of milk lipase to heat inactivation also varied with date. Lipolysis was also significantly affected by date. Cream free fatty acid levels were lower during the period of daytime grazing and were significantly higher than those in preheated milk. The correlation between lipase activity and free fatty acid levels was generally poor, accounting for between 0 and 34% of the variance. Possible reasons for the effect of separating temperature on lipolysis in cream are discussed.

Serum lipoprotein stimulation of lipolysis and its relevance to free fatty acid development in bovine milk

The role of blood serum lipoproteins in stimulating milk lipolysis was investigated by adding blood serum and heparin to the milk of normal cows and cows in which elevated somatic cell counts were induced by intramammary infusion of Escherichia coli endotoxin and Staphylococcus aureus. There was considerable variation between individual milks in the extent to which lipolysis was stimulated. In some milks there was almost no response. For values obtained from 29 cows during the first 6 months of lactation a relationship was observed between free fatty acid (FFA) levels in the untreated and serum-stimulated samples (r = 0·776). FFA values were higher after 18 h at 4 °C in milks from infused quarters than in those from control quarters, but this difference was not entirely due to higher values from the infused quarters at 0 h. Both blood serum and heparin stimulated lipolysis in high cell count milks and control milks. The response to heparin was greater than that to blood serum, but the response to both was highly correlated with FFA levels in the untreated milks. There were no differences in the ability of skim-milks prepared from infused and control quarters to promote lipolysis in recombined milks containing sterilized homogenized milk fat globules as a source of substrate. Incubation of blood serum with 2 μg/ml trypsin for 1 h at 37 °C reduced the ability of the serum to promote lipolysis. The importance of lipase activation by blood serum lipoproteins in relation to milk lipolysis is discussed.

Lipase redistribution in cows' milk during induced lipolysis. I. Activation by agitation, temperature change, blood serum and heparin

Effects of agitation, temperature change, blood serum and heparin on lipolysis and on distribution of lipoprotein lipase (LPL) in cold-stored cows' milk were measured. All individual milks (n = 135) were susceptible to activation treatments. With 'gentle' treatments, induced lipolysis was strongly correlated to spontaneous lipolysis (r greater than or equal to 0.74). In contrast, induced lipolysis in response to 'vigorous' treatments was more variable (0.7 greater than r greater than 0.34) and levelled off in milks with high spontaneous lipolysis. Induced lipolysis always resulted from an increase in LPL associated with cream. Strong correlations (0.79 less than or equal to r less than or equal to 0.94) were found between free fatty acid levels and cream LPL. All treatments (except heparin addition) also promoted lipolysis/unit cream LPL.

Effect of pregnancy, milk yield, and somatic cell count on bovine milk fat hydrolysis

During 12 mo, 1818 milk samples were collected from Holsteins and Jerseys (n = 261) to evaluate effects of advancing lactation and pregnancy on milk fat hydrolysis. Aliquots, cooled immediately and stored 48 h at 4 degrees C, were analyzed for free fatty acid content. Holsteins had higher acid degree values than Jerseys (.90 vs. .62). No difference in values was detected between alternate a.m. (.74) and p.m. (.76) sampling times. Repeatability of acid degree values from lactation to lactation was low (.22). Days in milk, days pregnant, and milk yield had curvilinear effects on acid degree values, whereas SCC effects were linear. Estimated acid degree value at 335 d in milk (average dry-off) was lowered from .80 to .63 when adjusted for days pregnant and to .48 when adjusted also for milk yield. These responses agree with the increased acid degree values associated with two late lactation events: increasing day pregnant and decreasing milk yield. Estrogen secreted by the developing fetal-placental unit could mediate changes in milk composition that promote milk fat hydrolysis.

Hydrolysis of bovine and caprine milk fat globules by lipoprotein lipase. Effects of heparin and of skim milk on lipase distribution and on lipolysis

Heparin can dissociate lipoprotein lipase from casein micelles, and addition of heparin enhances lipolysis in bovine but not in caprine milk. Heparin shortened the lag-time for binding of lipoprotein lipase to milk fat globules and for lipolysis. Heparin counteracted the inhibitory effects of skim milk on binding of lipase and on lipolysis. Heparin stimulated lipolysis in all bovine milk samples when added before cooling and in spontaneously lipolytic milk samples also when added after cooling. Heparin enhanced lipolysis of isolated milk fat globules. Hence, its effect is not solely due to dissociation of lipoprotein lipase from the casein micelles. Cooling of goat milk caused more marked changes in the distribution of lipase than cooling of bovine milk; the fraction of added 125I-labeled lipase that bound to cream increased from about 8 to 60%. In addition, caprine skim milk caused less inhibition of lipolysis than bovine skim milk. These observations provide an explanation for the high degree of cold storage lipolysis in goat milk. Heparin had only small effects on the distribution of lipoprotein lipase in caprine milk, which explains why heparin has so little effect on lipolysis in caprine milk. The distribution of 35S-labeled heparin in bovine milk was studied. In warm milk less than 10% bound to the cream fraction, but when milk was cooled, binding of heparin to cream increased to 45%.(ABSTRACT TRUNCATED AT 250 WORDS)

Iodine-125-labeled lipoprotein lipase as a tool to detect and study spontaneous lipolysis in bovine milk

The distribution of lipoprotein lipase among cream, casein, and milk serum can be evaluated by addition of a trace amount of 125I-labeled lipoprotein lipase to milk. Radioactive lipase was distributed in parallel to endogenous lipase under several conditions. In some milk samples, binding of lipase to cream increased when the milk was cooled. Correlation was good between bound labeled lipase and degree of cold-induced lipolysis in corresponding milk samples. Binding of lipase to cream or to casein was not saturable by addition of two-to threefold more lipase than is normally present in milk. In milk with a relatively high fraction of lipase bound to cream, a correspondingly lower fraction was associated with casein, whereas the fraction of lipase in milk serum was similar in all milk samples. Cold-induced binding of lipoprotein lipase to cream was not fully reversed when the milk was warmed again. Heparin released lipase from casein and increased the amount of lipase bound to cream after cooling.

Lipoprotein lipase activity of milk from cows with prolonged subclinical mastitis

The influence of prolonged subclinical mastitis on bovine milk lipoprotein lipase activity was investigated. Nine cows with at least one quarter with prolonged subclinical mastitis and at least one nonmastitic quarter were selected in various stages of lactation. Milk from subclinical quarters had a mean somatic cell count of 5.7 X 10(6) cells/ml while milk from nonmastitic quarters had an average somatic cell count of 9.4 X 10(4) cells/ml. Quarters with a subclinical infection contained the same pathogenic organisms for a minimum of 6 wk. The average milk lipoprotein lipase activity of 108.7 units/ml milk from subclinical quarters was 27.1% higher than the average enzyme activity of 79.2 units/ml milk from nonmastitic quarters. Conditions present in the mammary gland during prolonged subclinical mastitis could lead to increased milk lipoprotein lipase activity in raw milk.

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.