Inhibition of glyceraldehyde 3-phosphate dehydrogenase by plasma and serum ultrafiltrates due in part to a low-molecular-weight, nonpeptide material.

In an attempt to verify the existence in the blood of a diabetogenic peptide (somantin) derived from growth hormone, ultrafiltrates from plasma and serum from normal and diabetic subjects were prepared. The freeze-dried residues of these ultrafiltrates inhibited glyceraldehyde 3-phosphate dehydrogenase as somantin is claimed to do. However, the behavior of the inhibitory material on gel filtration on Sephadex G-10 indicated a molecular weight well below 700, rather than the considerably larger size claimed for somantin. The inhibitory material did not adsorb to Dowex 50W cation exchange resin at pH 2.5, while over 95 percent of ninhydrin-positive material was retained. Acid hydrolysis of the inhibitory material did not abolish its activity. Because of the presence of this low-molecular-weight, nonpeptide inhibitory material, inhibition of glyceraldehyde 3-phosphate dehydrogenase by a simple ultrafiltrate of plasma or serum is probably not a definitive measure of somantin.