Corticosteroids as effectors of lipid polymorphism of dielaidoylglycerophosphoethanolamine. A study using 31P NMR and differential scanning calorimetry.

Abstract

The influence of corticosteroids on the lipid polymorphism of dielaidoylglycerophosphoethanolamine was studied by 31P NMR spectroscopy and differential scanning calorimetry. Both techniques evidenced two transitions in the pure lipid samples. The first one corresponded to the gel----liquid crystalline phase transition. It occurred at a temperature of 38.9 degrees C, as measured by differential scanning calorimetry and at 35-40 degrees C as detected by 31P NMR. The second transition corresponded to the bilayer----hexagonal HII phase transition. It occurred at 64.2 degrees C as measured by differential scanning calorimetry and at 60 degrees C as detected by NMR. Addition of corticosteroids led to different specific effects on the bilayer----hexagonal HII phase transition, according to their chemical structure. These effects appear to be the result of low amounts of incorporated steroids, according to binding studies (partition coefficient values range between 5 and 54). The presence of a conjugated 3-keto group in the steroid molecule (progesterone) promoted a downward shift in the bilayer----hexagonal HII phase transition temperature by about 6 -7 degrees C as compared to the 3 beta-OH-bearing compound (pregnenolone), which did not exhibit any appreciable effect. No change in the delta H of transition could be measured. The presence of the 21-OH group (like in deoxycorticosterone) induced the formation of a structure, characterized by an isotropic lineshape of the 31P NMR spectrum at temperatures where the 'hexagonal' type of lineshape is present, without steroid. The transition from the bilayer to this other structure occurred at a slightly higher temperature than the bilayer----hexagonal HII phase transition. It corresponded to a peak in differential scanning calorimetry scans with a delta H of 2.1 kJ X mol-1. The presence of the 17 beta-OH group as present in 17 beta-OH-progesterone and 11-deoxycortisol suppressed the two former effects. These compounds had no influence on the bilayer----hexagonal HII phase HII phase transition. The additional presence of the 11 beta-OH group like in corticosterone and cortisol, evoked a stabilization of the bilayer organization as the bilayer----hexagonal HII phase transition temperature is shifted upward by about 10 degrees C. This was accompanied by a decrease of the delta H to 0.8 kJ X mol-1. Besides this, the corticosteroids did not affect to a large extent the gel----liquid crystalline phase transition: a general slight downward shift of the transition temperature and a small broadening of the transition were observed without significant change in the delta H.(ABSTRACT TRUNCATED AT 400 WORDS)