Date of Award:

5-1989

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences

Committee Chair(s)

C. Anthon Ernstrom

Committee

C. Anthon Ernstrom

Abstract

Bovine plasmin activity was measured on H-D-valyl-L-leucyl-L-lysyl-4-nitroanilide by following absorbance changes at 405 nm. Steady-state kinetic parameters Vmax, Km, KI, and KI' were estimated. Bovine plasmin is competitively inhibited by casein and has a Kcat of .0158 ΔA405/min/nM, Km of .107 mM substrate, and KI of .86 mg/ml casein. Bovine plasmin can be measured directly in bovine milk without interference from casein.

A total of 380 milk samples from nineteen Holstein (one herd) and nineteen Jersey (one herd) cows was collected monthly during one lactation period. Samples from each cow were analyzed for fat, protein, plasmin activity, plasminogen, pH, SCC, clotting time, curd firming rate, and final curd firmness. Three age groups form each breed/herd were chosen; first, third, and fourth and later lactations.

Plasmin activity in milk was most affected by lactation number, with milk from fourth- and later-lactation cows having higher activity than milk from first- or third- lactation cows. Plasmin activity in milk increased during lactation but was not affected by breed/herd, pH, protein, or fat. Plasminogen averaged 5.4 times the plasmin activity in milk and increased during the first five months of lactation. Plasmin activity was higher in milk collected ruing summer and fall but plasminogen was higher in milk collected during fall and winter. Percentage of the total (plasmin+plasminogen) enzyme activated to plasmin increased in late-lactation milk and in milk from fourth- and later-lactation cows.

Plasmin activity did not affect any milk clotting parameters in this study. Increased protein in milk resulted in shorter clotting times. When statistically adjusted for protein content, clotting time was longer in milk from the Holstein herd compared to the Jersey herd. Curd firming rate was increased in milk with higher protein and fat. Milk samples collected in the fall had faster firming rates than milk from other seasons. Firming rates remained constant during lactation but increased with higher protein and fat content. Jersey herd milk produced firmer curd than Holstein herd milk and milk collected in the fall had firmer curd than during the other seasons.

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