Casein, a powerful enhancer of the rate of plasminogen activation

Proteolytic Systems in Milk: Perspectives on the Evolutionary Function within the Mammary Gland and the Infant

Milk contains elements of numerous proteolytic systems (zymogens, active proteases, protease inhibitors and protease activators) produced in part from blood, in part by mammary epithelial cells and in part by immune cell secretion. Researchers have examined milk proteases for decades, as they can cause major defects in milk quality and cheese production. Most previous research has examined these proteases with the aim to eliminate or control their actions. However, our recent peptidomics research demonstrates that these milk proteases produce specific peptides in healthy milk and continue to function within the infant's gastrointestinal tract. These findings suggest that milk proteases have an evolutionary function in aiding the infant's digestion or releasing functional peptides. In other words, the mother provides the infant with not only dietary proteins but also the means to digest them. However, proteolysis in the milk is controlled by a balance of protease inhibitors and protease activators so that only a small portion of milk proteins are digested within the mammary gland. This regulation presents a question: If proteolysis is beneficial to the infant, what benefits are gained by preventing complete proteolysis through the presence of protease inhibitors? In addition to summarizing what is known about milk proteolytic systems, we explore possible evolutionary explanations for this proteolytic balance.

Extensive in vivo human milk peptidomics reveals specific proteolysis yielding protective antimicrobial peptides

Milk is traditionally considered an ideal source of the basic elemental nutrients required by infants. More detailed examination is revealing that milk represents a more functional ensemble of components with benefits to both infants and mothers. A comprehensive peptidomics method was developed and used to analyze human milk yielding an extensive array of protein products present in the fluid. Over 300 milk peptides were identified originating from major and many minor protein components of milk. As expected, the majority of peptides derived from β-casein, however no peptide fragments from the major milk proteins lactoferrin, α-lactalbumin, and secretory immunoglobulin A were identified. Proteolysis in the mammary gland is selective-released peptides were drawn only from specific proteins and typically from only select parts of the parent sequence. A large number of the peptides showed significant sequence overlap with peptides with known antimicrobial or immunomodulatory functions. Antibacterial assays showed the milk peptide mixtures inhibited the growth of Escherichia coli and Staphylococcus aureus . The predigestion of milk proteins and the consequent release of antibacterial peptides may provide a selective advantage through evolution by protecting both the mother's mammary gland and her nursing offspring from infection.

Cloning and characterization of a plasminogen-binding enolase from the saliva of the argasid tick Ornithodoros moubata

Significant amounts of enolase have recently been found in the saliva of the argasid tick Ornithodoros moubata, raising the question as to what the function of enolase in the tick-host interface is. Enolase is a multifunctional glycolytic enzyme known to act as a plasminogen receptor on cellular surfaces, promoting fibrinolysis and extracellular matrix degradation. Fibrinolysis could be important for ticks to dissolve clots that might be formed during feeding as well as to prevent clotting of the ingested blood meal in the tick midgut. Additionally, enolase-mediated extracellular matrix degradation could contribute to the tick feeding lesion. Moreover, previous observations suggested an additional antihaemostatic role for O. moubata enolase as a P-selectin antagonist ligand. Accordingly, the aim of the present study was to investigate the potential role of the O. moubata salivary enolase as a plasminogen receptor and P-selectin ligand, and to evaluate its potential as an antigen target for anti-O. moubata vaccines. The study included the cloning, sequencing and recombinant production of the O. moubata enolase, plasminogen binding and activation assays, P-selectin binding assays, animal immunization trials, and RNAi knockdown of the enolase gene. Here we confirmed that enolase is secreted to the saliva of the tick and provide convincing evidence for a role of this salivary enolase as a plasminogen receptor, most likely stimulating host fibrinolysis and maintaining blood fluidity during tick feeding. The RNAi experiments and immunization trials indicated that enolase could be also involved in the regulation of tick reproduction, suggesting new potential control strategies. Finally, the P-selectin binding experiments demonstrated that this enolase is not a P-selectin ligand.

Activity and Nature of Plasminogen Activators Associated with the Casein Micelle

In fresh milk, plasminogen, the zymogen form of plasmin (PL), is the predominant form. Therefore, plasminogen activators (PA) can contribute significantly to PL activity in milk. Both tissue-type PA (tPA) and urokinase-type PA (uPA) exist in milk; however, contradictory findings have been reported for which type of PA is most closely associated with the casein micelles. Little is known about the factors that might lead to variations in the individual activities of the PA. The objective of this work was therefore to investigate possible factors that might affect the association of tPA and uPA with the casein micelle and their activities thereafter. Plasminogen activators were isolated from milk samples with different somatic cell counts following 2 different isolation protocols. Determination of uPA, tPA, and PL activities was carried out quantitatively following chromogenic assays using 2 different substrates, and qualitatively using specialized sodium dodecyl sulfate-PAGE. Different isolation methods and conditions led to differences in uPA, tPA, and PL activities. Urokinase-type PA activity was significantly higher in PA fractions isolated from milk with high somatic cell counts than from milk with low somatic cell counts. Activity results indicated that in pasteurized milk uPA could dissociate from the somatic cells and bind to casein. Moreover, a high level of PL in isolated PA fractions contributed to significantly enhanced PA activities. Overall, results confirmed the association of both uPA and tPA with the casein micelle; however, their amounts, activities, and molecular weights varied based on the nature of the milk and methods of separation, with uPA being the PA with greater potential to affect plasminogen activation in milk.

Plasmin system and microbial proteases in milk: characteristics, roles, and relationship

Proteolysis of milk proteins can be attributed to both native proteases and the proteases produced by psychrotrophic bacteria during storage of fresh raw milk. These proteases cause beneficial or detrimental changes, depending on the specific milk product. Plasmin, the major native protease in milk, is important for cheese ripening. Milk storage and cheese-making conditions can affect the level of plasmin in the casein and whey fractions of milk. A microbial protease from a psychrotrophic microorganism can indirectly increase plasmin levels in the casein curd. This relationship between the plasmin system and microbial proteases in milk provides a means to control levels of plasmin to benefit the quality of dairy products. This paper is a short review of both the plasmin system and microbial proteases, focusing on their characteristics and relationship and how the quality of dairy products is affected by their proteolysis of milk proteins.

Plasminogen activation system in human milk

BACKGROUND

Plasmin is the major endogenous protease present in milk. The level of plasmin activity is controlled by the availability of the precursor plasminogen and by the levels of plasminogen activators and inhibitors. Recently, a differential distribution of tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) has been demonstrated in bovine milk. To assess whether this distribution pattern is a general feature, the occurrence of components of the plasminogen activation system in different fractions of human milk was investigated.

METHODS

Milk samples were separated into the following fractions; milk fat, skim milk, and milk cells by centrifugation. The different fractions were detected for the presence of plasminogen and plasminogen activators by immunoblotting and zymography. The distribution of t-PA and u-PA was investigated by ligand binding analysis. t-PA-catalyzed plasminogen activation was examined by a coupled chromogenic assay.

RESULTS

A differential distribution of plasminogen, t-PA, and u-PA was found. Casein micelles were found to exhibit t-PA and plasminogen binding activity, whereas the u-PA receptor was identified as the u-PA binding component in the cell fraction. Furthermore, human casein enhanced t-PA-catalyzed plasminogen activation, comparable to the enhancing effect obtained with fibrinogen fragments.

CONCLUSION

The finding of a differential distribution of u-PA and t-PA in milk suggests that the two activators may have different physiological functions, which involve protection against invading microorganisms and maintenance of patency and fluidity in the ducts of mammary gland, respectively.

Effect of individual caseins on plasminogen activation by bovine urokinase-type and tissue-type plasminogen activators

The effect was examined of individual caseins on the rate of plasminogen activation by bovine urokinase-type and tissue-type plasminogen activators. All individual caseins (alpha-CN, beta-CN, and kappa-CN) enhanced the activity of both types of plasminogen activators. Optimal concentrations for alpha-CN and beta-CN were 5 and 25 micrograms/ml, respectively. The enhancement of enzymatic activity declined when concentrations of alpha-CN and beta-CN were higher. In contrast, increasing concentrations of kappa-CN from 0 to 200 micrograms/ml resulted in corresponding increases in activity of both types of plasminogen activators. On a weight basis, alpha-CN was the most effective enhancer of plasminogen activator activity. Indirect evidence obtained with experiments utilizing alpha-CN immobilized on agarose suggested that the effect is related to extensive binding of plasminogen and both types of plasminogen activators to casein.

Distribution of plasminogen activator in different fractions of bovine milk

The type and relative amounts of plasminogen activator (PA) in different fractions of bovine milk obtained from 15 Holstein cows were examined. Raw milk was centrifuged to separate skim milk and a somatic cell pellet. PA was mainly localized within the casein fraction, being 42 times that in the serum, and in association with somatic cells. The predominant form of PA in milk casein was isolated from SDS-PAGE gel extracts and had a molecular mass of approximately 75 kDa. Its activity was increased 4.1-fold (P < 0.01) in the presence of fibrin but was unaffected by the presence of amiloride, indicating that it was due to tissue-PA. The predominant forms of PA associated with milk somatic cells were isolated from SDS-PAGE gel extracts and had molecular masses of approximately 30 and approximately 50 kDa. The activity of both proteins was unaffected by the presence of fibrin but was dramatically reduced by the presence of amiloride, indicating that they represented urokinase-PA.

Distribution of plasminogen activator forms in fractions of goat milk

Distribution of plasminogen activator forms in fractions of goat milk was examined. Raw milk was centrifuged to separate skim milk, cream, and a somatic cell pellet. Somatic cell extracts were obtained by sonication. Skim milk was centrifuged to separate milk serum and casein micelles. Activity of plasminogen activator was detected in casein, serum fractions, and in association with somatic cells. Plasminogen activator forms in milk casein had approximate molecular weights of 75,000, 50,000, and 30,000. The predominant forms of plasminogen activator in milk serum and in association with milk somatic cells had molecular weights of 30,000 and 50,000. Based on fibrin dependency and inhibition of activity in the presence of amiloride, the forms at 30,000 and 50,000 represent urokinase-plasminogen activator, and the form at 75,000 represents tissue-plasminogen activator.

The plasminogen activation system in bovine milk: differential localization of tissue-type plasminogen activator and urokinase in milk fractions is caused by binding to casein and urokinase receptor

We have analyzed the occurrence of components of the plasminogen activation system in bovine milk. Zymographic analyses showed that tissue-type plasminogen activator (t-PA) occurred in association with casein micelles, partially as a complex with type-1 plasminogen activator inhibitor (PAI-1), whereas urokinase-type plasminogen activator (u-PA) was confined to milk leukocytes. Whey contained a component with a plasminogen dependent proteolytic activity which was shown to be plasma prekallikrein (PPK). The u-PA in the milk leukocytes was shown to be bound to urokinase receptor (u-PAR). A purification to near-homogeneity of the bovine u-PAR was undertaken. Investigating the novel t-PA binding to casein micelles by ligand blotting and Sepharose immobilized casein, multimeric forms of kappa-casein and dimeric alpha s2-casein were identified as t-PA binding components. The kappa-casein gene and the fibrinogen gene are believed to have evolved from a common ancestor. Thus, the recent finding that casein enhances t-PA catalyzed plasminogen activation (Marcus, G., Hitt, S., Harvey, S.R. and Tritsch, G.L. (1993) Fibrinolysis 7, 229-236), and the observed t-PA/casein binding suggests that the casein micelle, which also contains plasminogen, may serve as a matrix for t-PA-catalyzed plasminogen activation in milk.