Serum adenosine deaminase in normals and in a patient with adenosine deaminase deficient-severe combined immunodeficiency

We have found 50% of normal adenosine deaminase (ADA) activity in the serum from a child with severe combined immunodeficiency (SCID) and genetic complete absence of ADA activity in other tissues (including erythrocytes, mononuclear and cultured cells). We, therefore, investigated properties of normal serum ADA and found that normal serum ADA activity is composed of two ADA isozymes with differing properties. Approximately 25-50% of normal serum ADA activity has properties similar to those of the major ADA isozymes (Ada₁ and ADA_1+CP) which are lacking in children with ADA deficient SCID. The remainder of serum ADA reflects activity of an isozyme with a molecular mass of approximately 100000 dalton. This isozyme has markedly different kinetic properties from those of the major ADA isozymes involved in genetic ADA deficiency. These include a markedly higher K_m for adenosine (2800 μmol/1), a markedly lower ability to deaminate deoxyadenosine than adenosine and a relative insensitivity to inhibition by erythro-9(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of the common ADA isozyme(s) (ADA₁ and ADA_{1 + CP}). The properties of this serum ADA isozyme are similar to those of the minor (~2%) ADA isozyme (ADA₂), present in similar amounts in tissues of ADA deficient patients and normals. These findings suggest that the 50% of normal ADA activity observed in serum of an ADA deficient child reflects activity of the 100000-dalton isozyme.