Development of mammalian sulfur metabolism: Absence of cystathionase in human fetal tissues

Abstract

Cystathionase activity was absent from human fetal liver and brain as early as 6 weeks of gestation. Hepatic methionine-activating enzyme (26 ± 3 nmoles/mg protein/hr) and hepatic cystathionine synthase (21 ± 4 nmoles/mg protein/hr) were present (cf. 86 ± 16 and 98 ± 19 nmoles/mg protein/hr, respectively, in mature human liver). All three activities were absent from the placenta. Human fetal liver contained higher concentrations of cystathionine (14 ± 2 /mmoles/100 g wet weight) than mature human liver (0) and human fetal brain (4.0 ± 0.6, mmoles/100 g wet weight). Methionine- activating enzyme of human fetal brain, but not liver, showed a tendency to increase with development (coefficient of correlation was 0.62; 0.01 < P < 0.05).35S-l-methionine injected into the umbilical vein of six human fetuses was incorporated into free methionine in liver and brain, but not into free cyst(e)ine, homocysteine, taurine or, except for the smallest fetus, cystathionine.35S-L-cysteine similarly injected was incorporated into free cysteine in liver and brain to a greater extent than in plasma, whereas it was incorporated into cystine in plasma to a greater extent than in either liver or brain. Incorporation of35S into cystathionine in liver was greater from36S-L-cysteine than from35S-L-methionine. Both35S-L-methionine and36S-L-cysteine were actively incorporated into tissue proteins: methionine > cysteine and liver > kidney > brain. Incorporation of35S-L.-methionine and35S-L-cysteine incubated with minced liver from four human fetuses showed more active incorporation of methionine (11,836— 15,045 dpm/mg protein) than cysteine (7,044-9,856 dpm/mg protein). © International Pediatrics Research Foundation, Inc. 1972. All Rights Reserved.