A New Variant of Serum Leucine Aminopeptidase in the Mouse: Its Development and Possible Regulation

Serum leucine aminopeptidase (LAP) isozymes were compared in four strains of inbred mice during postnatal development, adult life, and pregnancy. In pregnancy, no changes in the maternal serum LAP pattern were observed, in contrast to human studies. One strain, DD/S, differs from the other three in serum LAP. Polymorphism in serum LAP has not been previously described in the mouse. Neonatal DD/S mice exhibit a single band of serum LAP upon starch gel electrophoresis; however, between 14 and 18 days of age, two distinct bands appear, which persist throughout adult life. In the strains C57BL/6J, BALB/cJ, and DBA/2J there is a single band of activity at all stages. Crosses and backcrosses between DD/S and C57BL/6J show that the double-band variant is inherited as an autosomal recessive. The variant is independent of both the supernatant malic enzyme (Mod-1) and the intestinal LAP (Lap-1) loci, which are known to be linked on chromosome 9. The serum LAP variant is linked to an intestinal alkaline phosphatase variant. The presence of a separate structural gene is suggested by the genetic independence of the serum LAP variant from Lap-1. Also, the two serum LAP bands of DD/S are not interconverted by treatment with neuraminidase, beta-mercaptoethanol, or heat or by mixing the sera of DD/S and C57BL/6J prior to electrophoresis. The level of serum LAP activity in DD/S is approximately twice that in C57BL/6J. While these observations imply two structurally distinct proteins, the absence of any trace of the second LAP band in the heterozygote strongly suggests that the LAP variant protein is not the result of a separate structural gene. Intestinal LAP in DD/S migrates with the same electrophoretic mobility as the serum LAP variant, implying that the variant might originate in the intestine and its appearance in the serum be modulated by some factor at an unlinked locus.