Stimulation of the vitamin K-dependent carboxylase from bovine liver.

Vitamin K-dependent carboxylase from bovine liver is stimulated not only by reducing agents and bivalent metal ions (especially Mn2+), but also by several organic solvents (dimethyl sulphoxide, ketones and acetonitrile). The organic solvents stimulated both the carboxylation of glutamic acid residues and the formation of vitamin K epoxide. This stimulation by organic solvents was independent of the physical state of the phospholipid; it was highest at low temperatures and could only be demonstrated with vitamin K1 and not with 3-DTT-MK-O (the thioether adduct of menadione and dithiothreitol) or t-butyl hydroperoxide, which normally can substitute for vitamin K. We suggest that organic solvents exert their effect by changing the mobility of the isoprenoid side chain of vitamin K1 within the carboxylase complex

Isolation and partial characterization of a vitamin K-dependent carboxylase from bovine aortae.

Vitamin K-dependent carboxylase activity has been demonstrated in the crude microsomal fraction of the intima of bovine aortae. The procedure for the isolation of vessel wall carboxylase is a slight modification of the general preparation procedure for tissue microsomes. The highest activity of the non-hepatic enzyme was observed at 25 degrees C and hardly any NADH-dependent vitamin K reductase could be demonstrated. The optimal reaction conditions for both vessel wall as well as liver carboxylase were similar: 0.1 M-NaCl/0.05 M-Tris/HCl, pH 7.4, containing 8 mM-dithiothreitol, 0.4% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulphonic acid (CHAPS), 0.4 mM-vitamin K hydroquinone and 2 M-(NH4)2SO4. Warfarin inhibits the hepatic and non-hepatic carboxylase/reductase enzyme complex more or less to a similar degree. We have measured the apparent Km values for the following substrates: Phe-Leu-Glu-Glu-Leu ('FLEEL'), decarboxylated osteocalcin, decarboxylated fragment 13-29 from descarboxyprothrombin and decarboxylated sperm 4-carboxyglutamic acid-containing (Gla-)protein. The results obtained demonstrated that liver and vessel wall carboxylase may be regarded as isoenzymes with different substrate specificities. The newly discovered enzyme is the first vitamin K-dependent carboxylase which shows an absolute substrate specificity: FLEEL and decarboxylated osteocalcin were good substrates for vessel wall carboxylase, but decarboxylated fragment 13-29 and decarboxylated sperm Gla-protein were not carboxylated at all

Congenital deficiency of all vitamin K-dependent blood coagulation factors due to a defective vitamin K-dependent carboxylase in Devon Rex cats

Two Devon Rex cats from the same litter, which had no evidence of liver disease, malabsorption of vitamin K or chronic ingestion of coumarin derivatives, were found to have plasma deficiencies of factors II, VII, IX and X. Oral treatment with vitamin K1 resulted in the normalization of these coagulation factors. After taking liver biopsies it was demonstrated that the coagulation abnormality was accompanied by a defective γ-glutamyl-carboxylase, which had a decreased affinity for both vitamin K hydroquinone and propeptide. This observation prompted us to study in a well-defined in vitro system the possible allosteric interaction between the propeptide binding site and the vitamin K hydroquinone binding site on carboxylase. It was shown that by the binding of a propeptide-containing substrate to γ-glutamylcarboxylase the apparent K(M) for vitamin K hydroquinone is decreased about 20-fold. On the basis of these in vitro data the observed defect in the Devon Rex cats can be fully explained

Characteristics and composition of the vitamin K-dependent gamma-glutamyl carboxylase-binding domain on osteocalcin.

Two different sites on vitamin K-dependent gamma-glutamyl carboxylase (VKC) are involved in enzyme-substrate interaction: the propeptide-binding site required for high-affinity substrate binding and the active site for glutamate carboxylation. Synthetic descarboxy osteocalcin (d-OC) is a low-K(m) substrate for the VKC, but unique since it possesses a high-affinity recognition site for the VKC, distinct from the propeptide which is essential as a binding site for VKC. However, the exact location and composition of this VKC-recognition domain on d-OC has remained unclear until now. Using a stereospecific substrate analogue [t-butyloxycarbonyl-(2S,4S)-4-methylglutamic acid-Glu-Val (S-MeTPT)] we demonstrate in this paper that the high affinity of d-OC for VKC cannot be explained by a direct interaction with either the active site or with the propeptide-binding site on VKC. It is shown using various synthetic peptides derived from d-OC that there are two domains on d-OC necessary for recognition: one located between residues 1 and 12 and a second between residues 26 and 39, i.e. at the C-terminal side of the gamma-carboxyglutamate (Gla) domain. Both internal sequences contribute substantially to the efficiency of carboxylation. On the basis of these data we postulate the presence of a second high-affinity substrate-binding site on VKC capable of specifically binding d-OC, which is the first vitamin K-dependent substrate of which the VKC binding domain is interrupted by the Gla domain