A cAMP-triggered release of a hormone-like peptide

Preparations of the catalytic subunit of cAMP-dependent protein kinase from rabbit skeletal muscle, which appear to be homogeneous by SDS-polyacrylamide gel electrophoresis, were often found to contain a hormone-like factor (HLF) which causes an immediate rise, then a decline of intracellular cAMP in a B-lymphoma cell line. Active HLF is released when the fractions that contain it in an inactive form are incubated with cAMP prior to chromatography, or passed through an immobilized cAMP column. HLF seems to be a peptide: it loses its cell-stimulating capability after proteolysis and has an apparent molecular mass of 2.2-2.5 kDa.