Vol. 285, Issue 2, 753-758, May 1998

Stabilization of Vasoactive Intestinal Peptide by Lipids¹

Gennady Gololobov, Yasuko Noda, Simon Sherman, Israel Rubinstein, Janina Baranowska-Kortylewicz and Sudhir Paul

Departments of Anesthesiology (G.G., Y.N., S.P.) and Radiation Oncology (J.B.-K.) and Eppley Cancer Research Institute (S.S., S.P.), University of Nebraska Medical Center, Omaha, Nebraska, and Department of Medicine (I.R.), University of Illinois at Chicago, Chicago, Illinois

An anionic phospholipid, phosphatidylglycerol (PG), induced vasoactive intestinal peptide (VIP) to adopt a helical conformation, determined by circular dichroism studies. PG inhibited the trypsin-catalyzed, antibody-catalyzed and uncatalyzed cleavage of VIP, measured by radiometric and HPLC methods. Phosphatidylcholine, a neutral lipid, did not alter the circular dichroism spectra of VIP, and it was without detectable effect on the rates of VIP cleavage. Trypsin-catalyzed cleavage of Boc-Ile-Glu-Arg-methylcoumarinamide, a substrate unrelated in sequence to VIP, proceeded at equivalent rates in the absence and presence of PG, which suggests that the phospholipid did not exert a nonspecific inhibitory effect on the enzyme. Study of the kinetics of antibody-catalyzed VIP cleavage indicated that the inhibition by PG was due to decreased affinity for VIP, suggested by observations of increased K_m values and unaltered V_max values. Incorporation of VIP in the liposomes and the liposomal surface permitted maintenance of the peptide in essentially undegraded form at 37°C for 8 days. The longevity of liposomal VIP administered i.v. to mice was increased by about 5-fold compared with aqueous VIP. These observations indicate that certain phospholipids and liposomes can be applied to circumvent the rapid loss of VIP in vitro and in vivo due to degradative processes.