A Novel Leydig Cell cDNA-derived Protein Is a Relaxin-like Factor

According to Burkhardt et al. (Burkhardt, E., Adham, I. M., Brosig, B., Gastmann, A., Mattei, M. G., and Engel, W. (1994) Genomics 20, 13-19) Leydig cells contain the message for a protein of the insulin/relaxin family which was named Leydig cell insulin-like protein (LEY I-L). We have synthesized the human LEY I-L according to the amino acid sequence deduced from the published cDNA structure and obtained preliminary results concerning its potential target organs and its biological activity. Leydig cell insulin-like protein binds specifically to crude membrane preparations of mouse uterus and brain and shows cross-reactivity with the relaxin receptor, but not the insulin receptor. On the basis of these observations, together with the results of earlier structure-function considerations, we suggest that the new protein is a relaxin-like factor. By itself the new factor shows no obvious effect, but when given together with relaxin it significantly enhances relaxin-mediated widening of the mouse symphysis pubis.