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 tis sue sphingomyelinase activity. With [N-methyl-C-14]sphingomyelin as substra te, V-max for the enzyme in livers from control and mutant mice were, respe ctively, 29.6 and 11.6 nmol of substrate hydrolyzed h(-1) mg(-1) protein, a nd the corresponding K-m values were 94.6 and 132.3 mu M. The control and m utant enzymes showed similar pH profiles and temperature sensitivities. Whe n the control and mutant liver homogenates were mixed in various proportion s, the resulting activities were 70-80 % of the theoretical values. Cross a nd straight addition of total lipid extracts of control and mutant livers h ad minimal effect on their enzyme activities. The results suggest that the reduced sphingomyelinase activity of mutant liver is not due to the presenc e of inhibitors or absence of activators in this tissue.