Variation with season and lactation of plasmin and plasminogen concentrations in Montbeliard cows' milk

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Abstract
Summary: Plasmin and plasminogen were determined monthly over a one year period in samples of bulk and herd milks from Montbeliard cows. Montbeliard milk showed a high content of plasmin and plasminogen in comparison with milk from other breeds. In bulk milk, the plasmin activity reached a minimum in September (O·15 μg/ml milk) and a maximum in June (0·32 μg/ml milk). The annual mean concentration was 0·23 μg/ml milk. The plasminogen content varied around a value of 1·28 μg/ml milk, with a marked decrease in September (0·83 μg/ml milk) and a maximum in October (1·59 μg/ml milk). In herd milk, the minimum plasmin activity occurred in October (0·17 μ/ml milk) and the maximum in spring (O·42 μg/ml milk in May) with an annual mean of 0·30 μg/ml milk. The plasminogen content varied in a similar way, from 0·87 μg/ml milk to 1·82 μg/ml milk, with an annual mean of 1·46 μg/ml. The ratio plasminogen: plasmin ranged from 1·4 to 9·2 with an average of 4·9. From early to late lactation, plasmin and plasminogen concentrations increased from 0·25 to 0·38 μg/ml milk and from 1·07 to 2·01 μg/ml milk respectively and the plasminogen: plasmin ratio increased from 4·5 to 5·3. Studies of milk from cows at similar stage of lactation within a single herd have shown the necessity for distinguishing between two phases in the early stage of lactation. The first, a very early period, is usually present up to one month and the second occurs in the second and third months. Milk samples with the highest proportion of γ-caseins were not those with the greatest plasmin content but were those with high plasminogen contents, which had increased suddenly from the levels of the preceding month. This suggests a role for the plasminogen activator and inhibitor system.