Proteolysis of human apolipoprotein B: effect on quantitative immunoturbidimetry.

Apolipoprotein (Apo) B100 is the obligatory structural protein of low and very low density lipoproteins. We studied the appearance of a kallikrein-induced proteolytic fragment of Apo B100, identified by sodium dodecyl sulphate (SDS)-polyacrylamide electrophoresis as Apo B74, in plasma samples obtained from 7 men and 7 women. We also studied the effect of Apo B74 on total Apo B immunoreactivity by immunoturbidimetry. Apo B74 was found in plasma from 4 of the 7 women when the samples had been stored at 4 degrees C in glass or plastic tubes, but in some of the samples from the men. These 4 women used oral estrogens. The plasma samples which contained Apo B74 showed evidence of kallikrein activation. Presence of Apo B74 resulted in an underestimation of total Apo B immunoreactivity by 13% (P less than 0.05).