Metabolism of sphingosine bases, II. Studies on the degradation and transformation of [3-14C]erythro-DL-dihydrosphingosine,[7-3H]erythro-DL-sphingosine,[5-3H]threo-L-dihydrosphingosine and [3-14C;1-3H]erythro-DL-dihydrosphingosine in rat liver.