The assay of human plasminogen with casein as substrate

Abstract

When the proteolysis of casein by plasmin is followed by measuring the increase in material with an absorption at 276 m[mu] soluble in perchloric acid, a triphasic curve is obtained, the second part of which is linear. The assay of plasminogen preparations from single determinations after a fixed time of proteolysis gives unreliable results because of the influence of the lag period before the linear portion of the curve is reached. The length of the lag period before the linear portion of the reaction curve is reached is increased by increasing the substrate concentration or by decreasing the enzyme concentration. The slope of the linear part of the curve is proportional to enzyme activity and is almost independent of substrate concentration over a tenfold range. A standardized assay is described which gives reliable results, based on the slope of the linear part of the reaction curve.