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Turnover of Molecular Species of Sphingomyelin in Microsomes and Myelin of Rat Brain

The metabolism of sphingomyelins containing short‐chain fatty acids (C18:sphingomyelin) and those containing long‐chain fatty acids (C24:sphingomyelin) in microsomes and myelin from rat brain were studied for periods from 1 to 60 days after intracisternal injection of [32P]Na2HPO4. In microsomes the half‐lives of both molecular species of sphingomyelin were 14 days. In myelin the half‐life of C18:sphingomyelins was 69 days, whereas C24: sphingomyelins exhibit a high degree of metabolic stability. The results suggest that in myelin the sphingomyelins containing short‐chain fatty acids may be located in the inner leaflet of the bilayer and those containing long‐chain fatty acids may be located in the outer leaflet.