Sphingomyelinase activity of livers from control and NCTR-BALB/c mice

NCTR-BALB/c mice have an autosomal recessive disorder involving storage of sphingomyelin and unesterified cholesterol in their tissues and reduced tissue sphingomyelinase activity. With [N-methyl-14C]sphingomyelin as substrate, Vmax for the enzyme in livers from control and mutant mice were, respectively, 29.6 and 11.6 nmol of substrate hydrolyzed h(-1) mg(-1) protein, and the corresponding Km values were 94.6 and 132.3 microM. The control and mutant enzymes showed similar pH profiles and temperature sensitivities. When the control and mutant liver homogenates were mixed in various proportions, the resulting activities were 70-80% of the theoretical values. Cross and straight addition of total lipid extracts of control and mutant livers had minimal effect on their enzyme activities. The results suggest that the reduced sphingomyelinase activity of mutant liver is not due to the presence of inhibitors or absence of activators in this tissue.