6-Hexadecanoylamino-4-methylumbelliferyl-beta-D-galactopyranoside (HMGal) has 
been shown to be a specific fluorogenic substrate of galactocerebrosidase and to 
facilitate the simple enzymatic diagnosis of Krabbe disease in human patients 
and in twitcher mice. HMGal hydrolysis at pH 4.5 is optimally stimulated by 
sodium taurocholate (0.25%) and oleic acid (0.05%) with a Km of 0.150, 0.04 and 
0.03 mM, respectively for control mouse kidney, human fibroblasts and 
leukocytes. In control samples, the specific activity (nmol/mg prot./h) for 
HMGal is higher than for the natural substrate, galactocerebroside, and is 
severely deficient in the twitcher mouse and in patients with Krabbe disease. 
Comparative investigation of galactocerebrosidase activity in fibroblasts, 
leukocytes and brain with radioactive and fluorogenic substrates reveals a good 
agreement between the results of the two methods. Galactocerebroside (Gal-Cer) 
is a competitive inhibitor of HMGal hydrolysis in mouse kidney homogenates while 
GM1-ganglioside has no inhibitory effect in the same assay system. The 
sensitivity and specificity of this fluorogenic substrate for 
galactocerebrosidase provides a simple and rapid method for the diagnosis of 
Krabbe disease, and for the purification of this enzyme from normal tissues.