Adenosine deaminase activity of insect-derived growth factor is essential for its growth factor activity

Insect-derived growth factor (IDGF) was originally isolated from conditioned medium of NIH-Sape-4 cells derived from flesh fly embryos. Here we demonstrated that IDGF has adenosine deaminase activity. The substrate specificity of IDGF was similar to that of the mammalian cytoplasmic adenosine deaminase. The adenosine deaminase activity of IDGF was shown to be indispensable for its growth factor activity toward NIH-Sape-4 cells. We found that there are specific binding sites for IDGF on the surface of NIH-Sape-4 cells and that it binds to these sites with a K(d) value of 2.4 x 10(-10) m. We propose that the cell surface binding sites for IDGF are specific receptors modified with an adenosine moiety. When IDGF binds to these receptors, it may deaminate the adenosine moiety, and this process may be prerequisite for the signal transduction via this receptor.