ENZYMATIC HISTOCHEMISTRY OF THE TERM HUMAN PLACENTA

Kinetic and immunologic evidence for the absence of glucose-6-phosphatase in early human chorionic villi and term placenta

The existence of the enzyme glucose-6-phosphatase (G6Pase) in early and term human placenta was investigated by comparing the characteristics of placental microsomal glucose 6-phosphate (G6P) hydrolytic activity and liver G6Pase. Placental microsomes exhibited similar apparent Km values for G6P and beta-glycerophosphate in intact and deoxycholate-treated microsomes, heat stability at acidic pH, low latency of mannose 6-phosphate hydrolysis, very low activity of pyrophosphate: glucose phosphotransferase, and undetectable [U-14C]G6P transport into the placental microsomes, all of which indicated that specific G6Pase activity does not exist in placenta. Immunological evidence of the absence of both 36.5 kDa and T2 proteins, which represent the G6Pase catalytic protein and the phosphate/pyrophosphate transporter protein, respectively, confirmed that early and term human placenta are devoid of the multicomponent G6Pase enzyme.

Human placental 5'-nucleotidase: purification and properties

Subcellular fractionation of human term placenta showed that the highest relative specific activity of 5'-nucleotidase and alkaline phosphatase resided in the microsomal fraction; of the total 5'-nucleotidase activity present, 7 per cent was in the cytosol. 5'-Nucleotidase was reproducibly purified over 500-fold in 17 per cent yield from the insoluble component of homogenates of term placenta to give a single major glycoprotein with two minor inactive protein contaminants. Purified placental 5'-nucleotidase was free from non-specific or alkaline phosphatase, hydrolysed 12 to 22 mumol AMP/min/mg of protein at 30 degrees C, and was activated up to fivefold by Triton X-100. AMP, Km 5 to 7 microM, was the preferred substrate. The Arrhenius plot was biphasic, with activation energies of 21.7 and 49.7 kJ/mol above and below 36 degrees C, the region of the transition temperature. Nucleoside di- and triphosphates inhibited competitively; the most potent inhibitors were ADP and adenosine 5'-methylenediphosphonate, Ki slope 90 nm and 6 nm, respectively. Lectins inhibited the enzyme; concanavalin A caused time-dependent inactivation reversible by alpha-methyl-D-mannoside. EDTA inactivated the enzyme; partial reactivation was achieved with divalent cations. The pH optimum was 7.2 to 7.3; Mg2+ produced a second alkaline pH optimum. The properties of placental 5'-nucleotidase are those of an intrinsic membrane protein and, in general, resemble properties of the several 'ecto'-5'-nucleotidases which have been purified from other tissues, although certain differences in kinetic properties of the placental enzyme are apparent.

Enzyme histochemical patterns in the normal human amnion: amniotic histochemical patterns

The histochemical distribution of 14 enzymes in the human amnion at term are described. Adenosine triphosphatase, 5'-nucleotidase, acid phosphatase and nonspecific esterase show continuous gradations of enzyme activity. Cells with intense activity are prominent in acid phosphatase and nonspecific esterase preparations. A distinct subpopulation of amniotic cells can be defined by their alkaline phosphatase activity. The possible correlations with morphological studies are discussed.

An ultrahistochemical study of the distribution of acid and alkaline phosphatases in placentae from normal and complicated pregnancies

The subcellular localisation of acid and alkaline phosphatase has been studied in the trophoblast of placentae from both normal and complicated pregnancies. In placentae from uncomplicated pregnancies the number of trophoblastic acid-phosphatase-containing organelles decreases progressively as gestation proceeds whilst alkaline-phosphatase activity, although abundant at term, could not be demonstrated during the early stages of pregnancy. The acid-phosphatase-containing organelles are of two types; one is a small round body which is probably a lysosome whilst the other is a multivesicular body. The alkaline phosphatase is distributed mainly on the syncytial microvilli and plasma-membrane. It is suggested that the marked lysosomal activity during early pregnancy is related to the architectural refashioning of the placenta during this period and that there are two phosphatase-linked transfer systems in the trophoblast, one dependent upon acid-phosphatase-containing multivesicular bodies and being utilised during early pregnancy and the other reliant upon alkaline phosphatase and dominating during the second half of gestation. In placentae from prolonged pregnancies there is a further decrease in trophoblastic acid phosphatase and, usually, a continuing increase in alkaline-phosphatase activity. In placentae from babies of low birth weight this trend is sometimes reversed and alkaline-phosphatase activity either disappears or its reaction product diffuses throughout the syncytium; this is usually accompanied by a marked increase in the number of acid-phosphatase-containing multivesicular bodies. Placentae from women with pre-eclampsia show no loss of alkaline-phosphatase activity but are characterised by an increased number of lysosomal bodies.

Presence of adenosine in the human term placenta. Determination of adenosine content and pathways of adenosine metabolism

Adenosine, inosine, and hypoxanthine were isolated from human term placentas by paper chromatography of the dialyzates of aqueous placental homogenates and quantified by ultraviolet spectroscopy. Placentas obtained upon vaginal delivery contained 230 ± 18 nmoles adenosine/g tissue, 300 ± 29 nmoles inosine/g, and 250 ± 19 nmoles hypoxanthine/g. Three prelabor placentas obtained by cesarean section contained 44 ± 3 nmoles inosine/g and 277 ± 22 nmoles hypoxanthine/g, but adenosine was not detected in two of the placentas and was present only at 4 nmoles/g in the third. The activities of placental enzymes catabolizing ATP were determined in homogenates of term placentas. No adenylate deaminase or xanthine oxidase activity was found. Adenosinetriphosphatase, adenylate kinase, and 5'-nucleotidase were more active than were adenosine kinase, adenosine deaminase, and purine nucleoside phosphorylase. The results indicate that in the placenta both the formation of adenosine from adenosine monophosphate and the fate of adenosine, i.e., salvage or catabolism, are influenced by the level of adenosine triphosphate. Possibly, during labor, placental ischemia favors the enzymatic production of adenosine.

Specializations of the syncytium of the human chorion

Images