PHYSICOCHEMICAL CHARACTERIZATION OF PSYCHOSINE BY H-1 NUCLEAR-MAGNETIC-RESONANCE AND ELECTRON-MICROSCOPY

Krabbe's disease is an autosomal recessive disease that affects the ly sosomal enzyme galactosylceramidase. The storage of one of its substra tes, psychosine (beta-galactosylsphingosine), is thought to be respons ible for the induction of pathological changes. Psychosine has a free amine group which is necessary for the mediation of its toxic effects. in the present study, the physicochemical properties of psychosine we re investigated. Nuclear magnetic resonance (NMR) detected pH titratio n was used to determine that the amine group had a pKa of 7.18 +/- 0.0 5. Pulsed-field gradient NMR spectroscopy was used to determine that t he diffusion coefficient of 2.8 mM psychosine in D2O at pD 4.46 or 7.0 4 is 1.16 +/- 0.02 x 10(-10) m(2)/s or 0.77 +/- 0.02 x 10(-10) m(2)/s, respectively. Negative staining electron microscopy (EM) studies of a cidic and neutral solutions of psychosine also were performed. At pH 4 .5, spherical structures were formed, which were relatively stable bet ween 3, 120, and 216 h following preparation; the diameter ranged from similar to 14 nm at the earliest time point to similar to 18 nm at th e last time point. The critical micelle concentration (CMC) was 1.26 m M at pH 4.0. At pH 7.1, the structures changed from spherical structur es with a diameter of 15-23 nm, at the earliest time point, to a heter ogeneous population of structures ranging from spherical structures, w ith a diameter of only a few nm, to irregularly shaped oblong structur es that had one or more dimensions exceeding 100 nm. The NMR and EM da ta indicate that the deprotonation of the amine group causes psychosin e to form aggregates that are unstable, which prevents a determination of the CMC at a neutral pH. These data indicate that molecular intera ctions of psychosine at the acidic pH of the lysosome, where it is nor mally digested, are more orderly than those at the pH of the cytoplasm or extracellular space where psychosine goes during disease.