Unless secreted, lysosomal proteases mostly do not interact with milk proteins. so CCT007093 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 total 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. 1-antitrypsin). Open in a separate windows Fig. 1 An overview of the proteolytic system network in milk. Protease activator activity is usually depicted in green. Inhibitory activity is usually highlighted in CCT007093 reddish. Protein names in italics symbolize components of the proteolytic system not yet found in milk. u-PA: urokinase-type plasminogen activator; t-PA: tissue-type plasminogen activator We exhibited recently via mass spectrometry-based peptidomic sequencing that milk proteases release hundreds of peptides from proteins within the human and bovine mammary gland (27C30). The sequences of these peptides in term and preterm milk were analyzed with bioinformatic methods. These searches suggested that plasmin, cathepsin D, elastase, cytosol aminopeptidase and carboxypeptidase B2 are active in human milk throughout lactation (27, 31, 32). This combination of peptidomics and bioinformatic analysis shows with sequence-specific detail that even milk from healthy mammals milk begins to digest itself within the mammary gland (9, 32). However, in both human and bovine milk, the digested peptides represent only a minority of the total protein component, and only specific proteins CCT007093 are digested (27, 33). Certain proteins and fractions of proteins remain intact (lactoferrin, immunoglobulins, -lactoglobulin (for bovine milk)), while others are partially digested (caseins, osteopontin, polymeric immunoglobulin receptor). This obtaining raises the question: what purpose might this minimal, controlled degradation serve? The following summarizes what is known about the major proteolytic systems in milk. Plasmin system Plasmin, which cleaves around the experiments show that plasmin activity for a mixture CCT007093 of plasminogen and plasminogen activator is usually effectively increased by the addition of casein micelles (23, 48, 49). Both human and bovine caseins accelerate the rate of plasminogen activation by tissue-type plasminogen activator (49), likely due to Mouse monoclonal to CD4.CD4 is a co-receptor involved in immune response (co-receptor activity in binding to MHC class II molecules) and HIV infection (CD4 is primary receptor for HIV-1 surface glycoprotein gp120). CD4 regulates T-cell activation, T/B-cell adhesion, T-cell diferentiation, T-cell selection and signal transduction the proximity between the plasminogen and the plasmin activator (23). Interestingly, some of the plasminogen activator inhibitors, which inhibit the activators conversion of plasminogen to plasmin, are bound to tissue-type plasminogen activators in casein micelles (20). Ostensibly, the presence of the plasminogen activator inhibitors around the micelle prevents more considerable casein micelle degradation in the mammary gland. The other main type of plasminogen activator in milk, urokinase-type plasminogen activator is usually associated only with the human milk somatic cell portion (50), specifically the neutrophils (51, 52). The presence of -casein-derived peptides as the major degradation products in human and bovine milk (27, 30, 33), despite not being the most abundant protein, suggests that the active casein-bound plasmin degrades proteins that associate with the micelle structure. We hypothesize further that a major reason whey proteins such as -lactalbumin, secretory immunoglobulin A and lactoferrin do not yield digested peptides in milk (27) is because they do not associate with the micelles that contain the majority of active plasmin. Whey proteins globular structure and disulfide bonds also likely increase the resistance of these proteins to proteolysis in comparison with the looser structures of caseins. Cathepsin systems A number of cathepsins are present in milk, including cathepsins B (18, 19), D (18) and Z (18) in bovine milk and cathepsins D (13), B (15), H (41) and S (41) in human milk. Other cathepsins, including cathepsin L and cathepsin G may be present in bovine milk, but their presence has not been confirmed (3). This family is usually wide-ranging in functionality. Cathepsins, as a family, typically act within the lysosome at acid pH (53). A defining feature of cathepsins is usually that they can be inactivated by oxidation and reactivated by reducing brokers (glutathionine) (53). The dozen users of the cathepsin family have different structures and catalytic mechanisms (some are aspartic proteases, some are serine proteases and.