Galactosylsphingosine Galactosyl Hydrolase

Utilizing galactosylsphingosine (psychosine), specifically labeled with tritium at carbon 6 of galactose, as the substrate, activities of galactosylsphingosine galactosyl hydrolase were demonstrated in young adult rat brain. The radioactive reaction product was characterized as galactose. The pH optimum was 4.2 to 4.5. The reaction was linear for at least 5 hours and was proportional to the amount of the enzyme. The enzyme had a Km of 1.1 x 10-5 m. Oleic acid, sodium taurocholate, deoxycholate, and cholate were stimulatory for the activity of galactosylsphingosine galactosyl hydrolase. Tween 20 and Triton X-100 were inhibitory, while Myrj 59 (polyoxyethylene stearate) had no effect. Galactosylceramide was a competitive inhibitor with an inhibitor constant (Ki) of 2.0 x 10-5 m, a finding suggestive of the identity of galactosylsphingosine and galactosylceramide galactosyl hydrolases. These observations suggest that an alternate degradative pathway of brain galactosyl-ceramide through galactosylsphingosine might exist, in addition to the well established major degradative pathway through ceramide.