326 DAIRY CHEMISTRY AND BIOCHEMISTRY in milk and milk products; perhaps its pH optimum is too far removed from that of milk; it is also inhibited by inorganic phosphate Acid phosphomonoesterase(EC 3. 1.3.2). Milk contains an acid phospha tase which has a pH optimum at 4.0 and is very heat stable(LTLT pasteurization causes only 10-20% inactivation and 30 min at 88C is required for full inactivation). Denaturation of acid phosphatase under UHT conditions follows first-order kinetics. When heated in milk at pH 6.7 he enzyme retains significant activity following HTST pasteurization but does not survive in-bottle sterilization or UHT treatment. The enzyme is not activated by Mg2+(as is alkaline phosphatase), but it is slightly activated by Mn and is very effectively inhibited by fluoride. The level of acid phosphatase activity in milk is only about 2% that of alkaline phosphatase activity reaches a sharp maximum 5-6 days post-partum, then decreases and remains at a low level to the end of lactation Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography; purification up to 40000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 k Da and an isoelectric point of 7. 9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it a num ber of cha Although casein is a substrate for milk acid phosphatase, the major caseins, in the order as(as1 +as2)>B>K, also act as competitive inhibitors of the enzyme when assayed on p-nitrophenylphosphate, probably due to binding of the enzyme to the casein phosphate groups(the effectiveness of the caseins as inhibitors is related to their phosphate content) Significance. Although acid phosphatase is present in milk at a much wer level than alkaline phosphatase, its greater heat stability and lower pH optimum may make it technologically significant. Dephosporylation of casein reduces its ability to bind Cazt, to react with K-casein, to form micelles and its heat stability. Several small partially dephosphorylated peptides have been isolated from Cheddar and Parmesan cheese. However is not known whether indigenous or bacterial acid phosphatases are mainly responsible for dephosphorylation in cheese. Dephosphorylation may be rate-limiting for proteolysis in cheese ripening since most pro teinase and peptidases are inactive on phosphoproteins or peptides. It has been suggested that phosphatase activity should be included in the criteria for starter selection326 DAIRY CHEMISTRY AND BIOCHEMISTRY in milk and milk products; perhaps its pH optimum is too far removed from that of milk; it is also inhibited by inorganic phosphate. Acid phosphomonoesterase (EC 3.1.3.2). Milk contains an acid phospha￾tase which has a pH optimum at 4.0 and is very heat stable (LTLT pasteurization causes only 10-20% inactivation and 30min at 88°C is required for full inactivation). Denaturation of acid phosphatase under UHT conditions follows first-order kinetics. When heated in milk at pH 6.7, the enzyme retains significant activity following HTST pasteurization but does not survive in-bottle sterilization or UHT treatment. The enzyme is not activated by Mg2+ (as is alkaline phosphatase), but it is slightly activated by Mn2+ and is very effectively inhibited by fluoride. The level of acid phosphatase activity in milk is only about 2% that of alkaline phosphatase; activity reaches a sharp maximum 5-6 days post-partum, then decreases and remains at a low level to the end of lactation. Milk acid phosphatase has been purified to homogeneity by various forms of chromaotgraphy, including affinity chromatography; purification up to 40 000-fold has been claimed. The enzyme shows broad specificity on phosphate esters, including the phosphoseryl residues of casein. It has a molecular mass of about 42 kDa and an isoelectric point of 7.9. Many forms of inorganic phosphate are competitive inhibitors, while fluoride is a powerful non-competitive inhibitor. The enzyme is a glycoprotein and its amino acid composition is known. Milk acid phosphatase shows some similarity to the phosphoprotein phosphatase of spleen but differs from it in a number of characteristics. Although casein is a substrate for milk acid phosphatase, the major caseins, in the order cts(ctsl + ~1,~) > p > K, also act as competitive inhibitors of the enzyme when assayed on p-nitrophenylphosphate, probably due to binding of the enzyme to the casein phosphate groups (the effectiveness of the caseins as inhibitors is related to their phosphate content). Signijicance. Although acid phosphatase is present in milk at a much lower level than alkaline phosphatase, its greater heat stability and lower pH optimum may make it technologically significant. Dephosporylation of casein reduces its ability to bind Caz+, to react with K-casein, to form micelles and its heat stability. Several small partially dephosphorylated peptides have been isolated from Cheddar and Parmesan cheese. However, it is not known whether indigenous or bacterial acid phosphatases are mainly responsible for dephosphorylation in cheese. Dephosphorylation may be rate-limiting for proteolysis in cheese ripening since most pro￾teinases and peptidases are inactive on phosphoproteins or peptides. It has been suggested that phosphatase activity should be included in the criteria for starter selection