The overall activity of the leucocyte dchranching-enzyme system was determined in 31 relatives of patients with a deficiency of this system. The activity was determined by measuring the glucose production from phosphorylase limit dextrin by a previously described method and by a new, simplified method. Both methods gave essentially the same results. All relatives showed values within the normal range (0.12-0.80 mU/mg protein). Thus it is not possible to detect heterozygotes. This is in contrast with the observations of other authors who measured the incorporation of glucose into glycogen by the reverse reaction of amylo-l,6-glucosidase. Possible implications of the different results for the role of the components of the enzyme system are discussed. The fact that a new, unsuspected case of complete enzyme deficiency was found in one family stresses the importance of screening relatives of patients.
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