Levels of immunoreactive aldolase C, creatine kinase-BB, neuronal and non-neuronal enolase, and 14-3-3 protein in circulating human blood cells.

Five proteins found in the human nervous system have been measured by radioimmunoassay in human red cells, platelets, and lymphocytes. Two neuronal proteins (neurone-specific enolase and 14-3-3 protein) occur in platelets at levels equivalent to their concentration in brain, and in erythrocytes at levels approximately 10% of the level in brain. Two proteins characteristic of astrocytes in the cerebral cortex (creatine kinase BB and aldolase C) occur at low levels in platelets and are virtually undetectable in erythrocytes and lymphocytes. The more widely distributed non-neuronal enolase is present in erythrocytes, platelets and lymphocytes. The neurone-specific enolase and 14-3-3 protein immunoreactivities found in circulating blood cells have been characterised in terms of molecular mass, charge, and dilution characteristics in the respective radioimmunoassay and in each case appears to represent the intact protein. Controlled lysis of erythrocytes releases neurone-specific enolase and 14-3-3 protein in parallel with haemoglobin. The occurrence of brain proteins in circulating blood cells (which appears to be a species-dependent phenomenon) has the practical clinical consequence that minor degrees of especially red cell lysis can produce high serum levels of immunoreactivity. This represents a pitfall in the measurement of these proteins in serum as tumour markers or as indices of damage to the central nervous system.