Search

The gene encoding placental alkaline phosphatase (PLAP) displays a well-documented allelic polymorphism. Likewise, different phenotypes exist for the PLAP-related germ-cell alkaline phosphatase (GCAP). We investigated the extent to which various allelic GCAP positions are critical in determining the enzymatic, structural, and immunological properties of GCAP phenotypes. Three homozygous GCAP phenotypes [JEG3, BeWo, and wild-type (wt) GCAP] were analyzed and compared with a "core" GCAP mutant that contains the seven amino acid substitutions that are consistently different between PLAP and GCAP but are common to the three known allelic GCAP genotypes. Although some substitutions could influence the electrophoretic behavior of the phenotypes, the allelic differences did not affect the kinetic properties of GCAP. However, they did affect the immunoreactivity and conformation of the variants as detected with a panel of 18 epitope-mapped monoclonal antibodies (MAbs) to PLAP. The selective immunoreactivity of the PLAP/GCAP-discriminating MAb C2 was critically dependent on the nature of the allelic residues 133 and 361 in GCAP. Residue 133 was also important for the general stability of the molecule because BeWo and wt GCAP, which have Asn133 and Val133, respectively, instead of Met133, showed a consistently reduced heat stability compared to core GCAP and JEG3. Because the core GCAP mutant consistently shows the characteristics of wt GCAP, its use as an antigen should allow the generation of monoclonal antibodies to GCAP that will not cross-react with PLAP and whose immunoreactivity will only marginally be influenced by allelic GCAP variation.

Placental (PLAP) and germ-cell (GCAP) alkaline phosphatases are inhibited uncompetitively by L-Leu and L-Phe. Whereas L-Phe inhibits PLAP and GCAP to the same extent, L-Leu inhibits GCAP 17-fold more strongly than it does PLAP. This difference has been attributed [Hummer & Millán (1991) Biochem. J 274, 91-95] to a Glu----Gly substitution at position 429 in GCAP. The D-Phe and D-Leu enantiomorphs are also inhibitory through an uncompetitive mechanism but with greatly decreased efficiencies. Replacement of the active-site residue Arg-166 by Ala-166 changes the inhibition mechanism of the resulting PLAP mutant to a more complex mixed-type inhibition, with decreased affinities for L-Leu and L-Phe. The uncompetitive mechanism is restored on the simultaneous introduction of Gly-429 in the Ala-166 mutant, but the inhibitions of [Ala166,Gly429]PLAP and even [Lys166,Gly429]PLAP by L-Leu and L-Phe are considerably decreased compared with that of [Gly429]PLAP. These findings point to the importance of Arg-166 during inhibition. Active-site binding of L-Leu requires the presence of covalently bound phosphate in the active-site pocket, and the inhibition of PLAP by L-Leu is pH-sensitive, gradually disappearing when the pH is decreased from 10.5 to 7.5. Our data are compatible with the following molecular model for the uncompetitive inhibition of PLAP and GCAP by L-Phe and L-Leu: after binding of a phosphorylated substrate to the active site, the guanidinium group of Arg-166 (normally involved in positioning phosphate) is redirected to the carboxy group of L-Leu (or L-Phe), thus stabilizing the inhibitor in the active site. Therefore leucinamide and leucinol are weaker inhibitors of [Gly429]PLAP than is L-Leu. During this Arg-166-regulated event, the amino acid side group is positioned in the loop containing Glu-429 or Gly-429, leading to further stabilization. Replacement of Glu-429 by Gly-429 eliminates steric constraints experienced by the bulky L-Leu side group during its positioning and also increases the active-site accessibility for the inhibitor, providing the basis for the 17-fold difference in inhibition efficiency between PLAP and GCAP. Finally, the inhibitor's unprotonated amino group co-ordinates with the active-site Zn2+ ion 1, interfering with the hydrolysis of the phosphoenzyme intermediate, a phenomenon that determines the uncompetitive nature of the inhibition.

Alkaline phosphatase (AP) activity is stage specific in mouse embryos and may be associated with compaction and separation of trophectoderm from inner cell mass in preimplantation development. We previously sequenced a cDNA and two mouse AP genes that could contribute to the AP activity in embryos. Oligonucleotide primers were constructed from the three sequences and used in the reverse transcription-polymerase chain reaction technique to establish that two of the three AP isozymes are transcribed during preimplantation development. The predominant transcript (E-AP) is from a gene highly homologous to the human tissue-specific APs, but different from the mouse intestinal AP. Tissue non-specific (TN) AP also is transcribed, but there is approximately 10 times less TN-AP than E-AP transcript. The TN-AP isozyme is the predominant transcript of 7 to 14 day embryos and primordial germ cells. A switch in predominance from E-AP to TN-AP must occur during early postimplantation development. This study establishes a framework for experiments to determine the functions of the two isozymes during preimplantation development.