Fibrinogen Associated with Subcellular Platelet Particles

The subcellular localization of fibrinogen in platelets washed in an EDTA‐buffer medium has been studied with special reference to the granule‐bound fibrinogen. After sucrose density gradient centrifugation of platelet homogenates nine fractions of different densities were separated and studied for their relative contents of total protein, fibrinogen, β‐glucuronidase and acid p‐nitrophenyl phosphatase. Fibrinogen was found both in the soluble phase and in the particulate fractions. The distribution of fibrinogen calculated relative to lactic dehydrogenase activity showed that granule‐bound fibrinogen did not represent cytoplasmic contamination. The results further indicate a preferential association of granule‐bound fibrinogen with low‐density granules. The same was true for β‐glucuronidase and acid phosphatase. Presence of not only fibrinogen, but also of the two lysosomal enzymes, in high amounts in the soluble fraction indicate that fibrinogen in this fraction may have been derived from disrupted granules.