The activation of coagulation factor X. Identity of cleavage sites in the alternative activation pathways and characterization of the COOH-terminal peptide.

Bovine Factor X can be activated by two alternative pathways. The first, favored at high concentrations of the complex of tissue factor and Factor VII, is initiated by the action of Factor VII on Factor X to cleave an activation peptide from the NH2 terminus of the heavy chain, to produce alpha-Xa. This is then converted autocatalytically to another form of Factor Xa, beta-Xa, by the loss of a 17-residue glycopeptide from the COOH terminus of the heavy chain, in a lipid-dependent reaction. The alternative pathway, favored at lower activator concentrations, is initiated by the action of Factor Xa on Factor X, in the presence of lipid, to release the same COOH-terminal peptide as is produced in the conversion of alpha-Xa to beta-Xa. The intermediate produced by the loss of this peptide from Factor X,I1, can be activated directly to beta-Xa by the tissue factor-Factor VII complex, with the loss of the same NH2-terminal peptide as is produced in the conversion of Factor X to alpha-Xa. The autocatalytic activation of Factor X by Factor Xa described previously occurs to a marked extent only at very low activator concentrations, and has been shown to proceed largely by the loss of the normal NH2-terminal peptide from the heavy chain of I1-Initial experiments show that neither peptide affects the rate of coagulation by either the extrinsic or intrinsic pathways. The amino acid sequences have been determined on both sides of the peptide cleavages, and it has been shown that the cleavage sites are the same, regardless of the pathway of activation. The amino acid sequence and carbohydrate composition of the COOH-terminal peptide have been determined. The carbohydrate moiety is attached via an O-glycosidic linkage at a threonine residue, and contains galactosamine but no glucosamine.     

L-lactate transport in Ehrlich ascites-tumour cells.

Ehrlich ascites-tumour cells were investigated with regard to their stability to transport L-lactate by measuring either the distribution of [14C]lactate or concomitant H+ ion movements. The movement of lactate was dependent on the pH difference across the cell membrane and was electroneutral, as evidenced by an observed 1:1 antiport for OH- ions or 1:1 symport with H+ ions. 2. Kinetic experiments showed that lactate transport was saturable, with an apparent Km of approx. 4.68 mM and a Vmax. as high as 680 nmol/min per mg of protein at pH 6.2 and 37 degrees C. 3. Lactate transport exhibited a high temperature dependence (activation energy = 139 kJ/mol). 4. Lactate transport was inhibited competitively by (a) a variety of other substituted monocarboxylic acids (e.g. pyruvate, Ki = 6.3 mM), which were themselves transported, (b) the non-transportable analogues alpha-cyano-4-hydroxycinnamate (Ki = 0.5 mM), alpha-cyano-3-hydroxycinnamate (Ki = 2mM) and DL-p-hydroxyphenyl-lactate (Ki = 3.6 mM) and (c) the thiol-group reagent mersalyl (Ki = 125 muM). 5. Transport of simple monocarboxylic acids, including acetate and propionate, was insensitive to these inhibitors; they presumably cross the membrane by means of a different mechanism. 6. Experiments using saturating amounts of mersalyl as an "inhibitor stop" allowed measurements of the initial rates of net influx and of net efflux of [14C]lactate. Influx and efflux of lactate were judged to be symmetrical reactions in that they exhibited similar concentration dependence. 7. It is concluded that lactate transport in Ehrlich ascites-tumour cells is mediated by a carrier capable of transporting a number of other substituted monocarboxylic acids, but not unsubstituted short-chain aliphatic acids.     

Effect of lithium on hypothalamic-pituitary-thyroid function in patients with affective disorders.

Hypothalamic-pituitary-thyroid (H.P.T.) function was assessed in 17 patients on maintenance doses of lithium carbonate for a mean period of 21 months (range 1-67 months) and by serial studies on four patients from the start of lithium treatment for a maximum of six months. An exaggerated thyrotrophin (TSH) response to intravenous thyrotrophin-releasing hormone (TRH) occurred in 14 of the 17 patients on maintenance treatment, though basal TSH levels were raised in only three. Two of the three patients were clinically and biochemically hypothyroid and showed a delayed recovery of normal H.P.T. function after lithium was stopped. There were no significant differences in thyroid hormone or basal TSH levels between the euthyroid lithium-treated.     

Differential use of zooplankton prey by Ancient murrelets and Cassin's auklets in the Queen Charlotte Islands

The distribution at sea and the food of two similar sized plankton-feedingalcids were examined during the 1981 breeding seasons in thenorthwestern Queen Charlotte Islands, British Columbia. Thetwo alcids, the Ancient murrelet (Synthliboramphus antiquus)and the Cassin's auklet (Prychoramphus aleuticus) have differentchick-rearing strategies. Both species fed predominantly atthe shelf break, although the Cassin's auklet also foraged overseamounts. The feeding distributions of the species appear tobe related to those of their prey. Zooplankton sampling indicatedthat each alcid selects a small and different portion of thezooplankton available in surface waters. The Ancient murrelet'smain foods were euphausiids (Thysanoessa spinifera and Euphausiapacifica) and larval and juvenile fishes. The Cassin's aukletchicks fed chiefly on calanoid copepods (Neocalanus cristatus).euphausiids (mostly Thysanoessa longipes in 1981, but in otheryears also Thysanoessa spinifera), and larval and juvenile fishes.The Cassin's auklets took smaller prey than the Ancient murrelet.Differences in the diets of the two alcid species were associatedwith differences in morphology and chick-rearing strategies.     

Prenatal screening for trisomy 18 with free beta human chorionic gonadotrophin as a marker.

OBJECTIVE--To determine the relation between maternal serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations in pregnancies complicated by trisomy 18 and establish whether prenatal biochemical screening for this condition could be developed in a way similar to that proposed for trisomy 21. DESIGN--Serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations in women with singleton pregnancies affected by cytogenetically confirmed trisomy 18, uncomplicated by neural tube defect or ventral wall defect, were identified from prospective trisomy 21 screening programmes. Additionally, stored maternal serum from similar pregnancies was analysed retrospectively. Analyte concentrations from singleton unaffected pregnancies were identified from a prospective screening programme as controls. Statistical parameters of the affected and unaffected populations were compiled. SETTING--Biochemical screening laboratories in Britain and the United States. SUBJECTS--52 women with singleton pregnancies complicated by trisomy 18; control population of 6661 women with unaffected singleton pregnancies. MAIN OUTCOME MEASURES--Median values of each analyte and their distribution in the affected and unaffected populations; detection rate of trisomy 18 and the false positive rate. RESULTS--Maternal serum alpha fetoprotein and free beta human chorionic gonadotrophin concentrations were significantly lower in pregnancies complicated by trisomy 18 (median values 0.71 and 0.37 respectively). By using a multivariate risk algorithm incorporating maternal age risk of trisomy 18 and the concentration of the two biochemical markers it was predicted that 50% of trisomy 18 cases (unaffected by neural tube defect or ventral wall defect) could be detected with a 1% false positive rate. CONCLUSION--Second trimester biochemical screening for trisomy 18 could be a valuable addition to trisomy 21 screening programmes.