by one group136 differed by a factor of two from the mean activities observed by another group.128 Technical aspects of the assays (especially the substrates used) and handling or storage of samples likely explain these reported differences.
Hexosaminidase seems to have the highest specific activity of the lysosomal enzymes in AF.128 Except for α‐glucosidase, α‐arabinosidase, and β‐glucosidase, lysosomal enzymes generally rise to their highest specific activities at term.134 The specific activities of α‐glucosidase and heat‐labile alkaline phosphatase reach a peak of specific activity between 13 and 18 weeks of gestation. Prenatal diagnosis of metachromatic leukodystrophy requires assay of arylsulfatase A enzyme activity in cultured AF or chorionic villus cells, or DNA analysis if the mutation is known.137 Higher than normal activities of several lysosomal hydrolases were reported in the AF of a fetus affected with I‐cell disease (mucolipidosis II).138, 139 All enzyme diagnostic tests based on cell‐free AF should be used with caution.
In some specific inborn errors of metabolism, such as Tay–Sachs disease, the characteristic enzymatic deficiency (hexosaminidase A) may manifest in the AF.140, 141 Desnick et al.142 found one fetus affected with Sandhoff disease (total hexosaminidase deficiency) with almost complete deficiency of this enzyme in the AF. This finding was confirmed in another Sandhoff‐affected fetus.143 Potier et al.143 found that the AF samples with high total hexosaminidase activity also contained a high percentage of maternal serum hexosaminidase (form P). The varying rates of enzyme inactivation in AF and the possibilities of maternal or fetal serum contamination or maternal tissue admixture of different isozymes, in addition to points already made, confirm that enzyme assays performed directly on cell‐free AF are unreliable. Thus, direct study of enzyme activity in chorionic villi or cultivated AF cells is preferable.144
Amino acids
Studies on amino acid concentrations in fetal tissues and AF were initiated when prenatal diagnosis was introduced145, 146 (see Chapter 22). Dallaire et al.145 measured the concentration of amino acids and related compounds in 111 samples of AF and 89 maternal plasmas between the 10th and 40th weeks of pregnancy. The concentration of eight amino acids decreased toward the end of the pregnancy, whereas 13 amino acids showed no significant change between 10 and 40 weeks, and 10 were present in trace amounts. Variation in lysine values between10 and 20 weeks did not permit fetal age correlation studies. There was a marked elevation of amino acid concentrations in AF obtained from sacs containing two fetuses. Matched AF and maternal plasma samples, studied between 10 and 17 weeks, showed no significant correlation. Elevated levels of homocysteine were noted in the second trimester independent of the methylene tetrahydrofolate reductase genotype.147
It had been postulated that the AF, being at first an isotonic transudate from the maternal plasma, may become hypotonic with the increase in fetal urine. A dilution factor could explain a general decrease in total amino acid concentration toward term, and the increase in urea and creatinine could come from the maturation of the urinary system. However, a change in fetal metabolism may explain the higher concentration of some amino acids during the end of pregnancy.
The concentrations of amino acids were measured in samples of coelomic fluid obtained from normal pregnancies between 7 and 12 weeks of gestation.148 The total molar concentration of the 18 amino acids measured was 2.3 times higher in coelomic fluid than in maternal serum, suggesting that levels of amino acids are influenced by placental synthesis and do not depend on maternal amino acid metabolism. Levels of amino acids were significantly higher in coelomic fluid than in AF, perhaps to support the metabolism of the secondary yolk sac.
Jauniaux et al.149 measured the distribution of amino acids between 7 and 11 weeks of gestation in samples of coelomic and AF, maternal serum, and homogenates of placental villi. They found a significant positive relation between maternal serum and placental tissue for ten amino acids, indicating that active amino acid transport and accumulation by the human syncytiotrophoblast occurs as early as 7 weeks. The concentration distributions of individual amino acids in coelomic and AF were related, indicating a passive transfer through the amniotic membrane. Later, these authors146 measured the concentration of 23 free amino acids in homogenates of fetal liver and samples of fetal plasma from 20 pregnancies between 12 and 17 weeks and compared those with matched samples of maternal plasma and AF. A fetomaternal plasma concentration gradient was observed for 21 amino acids, indicating that the fetomaternal amino acid gradient across the placenta is established from very early in pregnancy. The amino acid concentration pattern was similar in fetal plasma and AF but different in fetal liver, supporting the concept that it is essentially placental transport and metabolism that provides the fetus with these molecules.
Measurements of amino acids between the 13th and 23rd weeks of gestation showed that the concentrations of Ala, Lys, Val, Glu, Pro, Thr, and Gly accounted for about 70 percent of the amino acids in AF.150 A negative correlation with gestational age was found for Leu, Val, Ile, Phe, Lys, Ala, Asp, Tyr, Glu, and Pro. The concentration of Gln increased slightly, whereas the other amino acids did not change significantly during this period. Statistically significant positive correlations, at all gestational ages, were observed among Val, Leu, and Ile. These branched‐chain amino acids also correlated positively with Phe, Lys, Asp, Thr, Ser, Glu, Pro, Gly, Ala, and Tyr, and the amino acids within this group correlated with each other. In addition, strong positive correlations were observed between Phe and Tyr and between Gly and Ser.
AF amino acid levels are not influenced by normal variations in maternal amino acid concentrations.151 However, if the mother has an enzyme deficiency, a specific amino acid may be found in high concentration in the AF. Observation of a constant phenylalanine/tyrosine ratio in fetal AF supports the hypothesis that phenylalanine hydroxylase is present from the ninth week of pregnancy. The prenatal diagnosis of phenylketonuria (PKU) is now based on a molecular study (see Chapter 14).
An increased level of AF citrulline152 and an abnormal citrulline/ornithine + arginine ratio153 have been observed in argininosuccinate synthetase deficiency, although molecular testing is the recommended assay for prenatal diagnosis of citrullinemia154–156 (see Chapter 22).
During the second and third trimesters, galactitol accumulates in the AF and tissues of fetuses affected with galactosemia or clinical variant galactosemia.157, 158 Molecular genetic testing of chorionic villus sampling (CVS) or AF cells is preferred (when parental mutations are known) over enzyme analysis.158
Coude et al.159 reported that methylmalonic and propionic acidemia could be diagnosed during the first trimester of pregnancy. Jakobs et al.160 reviewed the usefulness of metabolite determinations in AF samples to diagnose amino and organic acidurias. Tyrosinemia type I and propionic acidemia have been diagnosed at the end of the first trimester via amniocentesis. One interesting finding related to amino acid metabolism has been the demonstration of succinylacetone in the AF of fetuses affected with hereditary tyrosinemia type I secondary to a deficiency of fumarylacetoacetate hydrolase in the liver.161, 162 Prenatal diagnosis of tyrosinemia type I involving the measurement of succinylacetone in AF at 12 weeks of gestation has been offered to couples at risk since 1982,162 but may not be reliable.163 Affected fetuses and heterozygote carriers can now be identified by DNA analysis, including for tyrosinemia type II.164
Disaccharidases
With the exception of lactase, which develops only a few weeks before term, disaccharidases are fully developed in the human fetal intestine as early as 10 weeks.165 The fetal kidney contains only trehalase and some maltase activities in detectable quantities. The intestinal mucosa contains disaccharidases able to hydrolyze a variety of substrates.