Heat resistance, immunological and quantitative changes of neutrophil alkaline phosphatase in trisomy 21 pregnancies

Maternal serum urea resistant alkaline phosphatase in Down syndrome pregnancy

BACKGROUND

The normal levels of alkaline phosphatase (AP) activity in maternal serum are virtually the same as those observed in Down syndrome (DS) pregnancies at 14-20 weeks' gestation. Using urea inhibition of AP, we observed an atypical AP isoenzyme in the neutrophils of mothers with trisomy 21 fetuses.

AIM

To assess the use of urea as a selective inhibitor of serum AP in order to seek a possible diagnostic difference between normal and DS pregnancies.

STUDY DESIGN AND SUBJECTS

Serum AP samples from 24 DS pregnancies and 204 control cases were examined at 12-22 weeks' gestation with and without 2.5 M urea AP inhibition at 18 degrees C for 2 h. The levels of AP activity obtained without urea and the percentage urea AP inhibition were analyzed in the two groups.

RESULTS

Without urea treatment, no significant difference of total alkaline phosphatase activity levels was detected between the 204 normal controls and the 24 DS samples. Using 2.5 M urea AP inhibition, after 120 min of exposure at 18 degrees C, the residual activity, as a percentage of initial values of AP, showed significantly higher resistance in the DS samples (> or = 50 IU/l of total AP activity) at 15-22 weeks' gestation. However, at 12-14 weeks (< or = 45 IU/l of total AP activity), no significant difference was found between the DS and control cases.

CONCLUSION

Serum urea resistant alkaline phosphatase in DS pregnancies showed a significant difference only at 15-22 weeks' gestation, compared with normal controls.

Kinetic comparison of tissue non-specific and placental human alkaline phosphatases expressed in baculovirus infected cells: application to screening for Down's syndrome

BACKGROUND

In humans, there are four alkaline phosphatases, and each form exhibits a characteristic pattern of tissue distribution. The availability of an easy method to reveal their activity has resulted in large amount of data reporting correlations between variations in activity and illnesses. For example, alkaline phosphatase from neutrophils of mothers pregnant with a trisomy 21 fetus (Down's syndrome) displays significant differences both in its biochemical and immunological properties, and in its affinity for some specific inhibitors.

RESULTS

To analyse these differences, the biochemical characteristics of two isozymes (non specific and placental alkaline phosphatases) were expressed in baculovirus infected cells. Comparative analysis of the two proteins allowed us to estimate the kinetic constants of denaturation and sensitivity to two inhibitors (L-p-bromotetramisole and thiophosphate), allowing better discrimination between the two enzymes. These parameters were then used to estimate the ratio of the two isoenzymes in neutrophils of pregnant mothers with or without a trisomy 21 fetus. It appeared that the placental isozyme represented 13% of the total activity of neutrophils of non pregnant women. This proportion did not significantly increase with normal pregnancy. By contrast, in pregnancies with trisomy 21 fetus, the proportion reached 60-80% of activity.

CONCLUSION

Over-expression of the placental isozyme compared with the tissue-nonspecific form in neutrophils of mother with a trisomy 21 fetus may explain why the characteristics of the alkaline phosphatase in these cells is different from normal. Application of this knowledge could improve the potential of using alkaline phosphatase measurements to screen for Down's syndrome.

Inhibition by sodium thiophosphate and L-p-bromotetramisole of neutrophil alkaline phosphatase in normal and trisomy 21 pregnancies

Alkaline phosphatase concentrations are known to increase in blood neutrophils of normal pregnant women. The main kinetic parameters of this enzyme were analysed and compared in a group of 30 women with normal pregnancies and a group of 11 women whose fetuses had trisomy 21 (Down's syndrome = DS). The subjects were studied at an identical stage of gestation. Significant changes occurred in thermal stability and urea resistance in cases of DS pregnancies. We also investigated the inactivation constants for two chemicals: L-p-bromotetramisole, an uncompetitive inhibitor, and sodium thiophosphate, a competitive inhibitor. Ki measured for the two inhibitors were found to be significantly lower in cases of pathological pregnancies. The patterns observed in inhibition constants extend the biochemical characteristics of the atypical isoenzyme expressed in neutrophils of women with DS pregnancies.

Difference in activity properties and subcellular distribution of neutrophil alkaline phosphatase between normal individuals and patients with trisomy 21

Biochemical, cytochemical characteristics and electron microscopy subcellular distribution of neutrophil alkaline phosphatase (NAP) were analysed in blood and/or smear samples from 39 trisomy 21 patients (Down's syndrome) aged 11.5-18 years (mean 15.5 years) and 55 normal subjects aged 12-20.5 years (mean 17 years). All patients were karyotyped. NAP cytochemical procedures were carried out on all subjects; biochemical NAP determinations were made in 10 patients and 20 controls; ultrastructural electron microscopy of AP was performed in three patients and four normal subjects. Neutrophil alkaline phosphatase from patients with trisomy 21 displayed the following changes: (1) a significant increase of enzyme activity, (2) a high thermal lability of enzyme. Electron microscope morphology exhibited large deposits of NAP reaction product associated with the plasma membrane and intracellular main organelles, like phosphasomes. The NAP biochemical and cytochemical characteristics suggest that trisomy 21 neutrophils contain a non-specific AP isoenzyme, closely related to the early placental form.