Activities of Enzymes Involved in Amino‐Acid Metabolism in Developing Rat Placenta

Abstract

Aspartate transaminase, alanine transaminase, glutamate dehydrogenase, arginase, serine dehydratase, tyrosine transaminase, glutamine synthetase, glutaminase and adenylate deaminase activities were measured in crude homogenates of 12, 19 and 21-day rat placentae. There is a considerable quantitative importance in enzymes able to produce free ammonia, such as adenylate deaminase and glutamate dehydrogenase, activity that progressively decrease with the age of placenta. The glutamine synthetase and tyrosine transaminase activities increase with age, while serine dehydratase decreases considerably and aspartate and alanine transaminase do not change practically. Arginase shows a maximum at 19, with lower 12 and 21-day activities. No measurable glutaminase activity has been found.The possible implications of the enzymes studied upon the ammonia-producing activity of rat placenta are discussed together with the relative decreasing role of placenta for the overall metabolic activity of the foetus, especially during the last phases of its development.The placenta is the interface through which the embryo first, and then the foetus, receive. the foodstuffs needed for their development at the expense of the maternal organism. The vital cycle of the placenta is related to its size relative to that of the foetus; it initially carries on many metabolic functions that are later taken up by other foetal organs, ending its physiological use with birth, when it is totally discarded.The metabolic functions of the placenta are manifold in addition to its main connecting and transporting role. Thus, the enzyme activities found in placenta are considerable, and explain an active glucidic and intermediary metabolism [l, 21; however, the amino acid metabolism of the placenta has been only sparsely studied, mainly in relation to transaminases [I -51 ; but also adenylate deaminase [6] and arginase [2,7] have been isolated and/or studied. Human placental gluconeogenic amino acid oxidation has been also described [8].

Expression and Role of SNAT3 in the Placenta

“…The mRNAs of SNAT3 were found to be expressed in Bewo cells Eadie-Hofstee plot analysis showed that glutamine uptake was two-phase uptake (high-affinity phase-0.13mM, low-affinity phase-1.4mM).We suggest that Km value of SNAT3 was similar to that of low affinity phase and SNAT3 transport glutamine in cytotrophoblast cells. In the rat placenta, the activity of glutamine synthetase is greatest on the 12 th day of gestation [40]. This corresponds to our finding that the expression level of SNAT3 in the placenta is the highest in the 11 th day of gestation.…”

Lactate Production and Absence of Gluconeogenesis from Placental Transferred Substrates in Fetuses from Fed and 48-H Starved Rats

“…Recently we reported the ability of rat placenta to synthesize lactate from both D-glucose (31) and L-alanine (32). Production of placental lactate from L-alanine seems to be involved in the mechanism of the maternofetal transfer of the amino acid (32) and is consistent with the reported presence of alanine aminotransferase in rat placenta (16,33). The level of labeled lactate in fetal plasma after infusion through the maternal left uterine artery was much lower with [U-14C]-glycerol than with the other substrates studied.…”

The effect of dietary protein on the amino acid supply and threonine metabolism in the pregnant rat

“…It has been reported that the developing placenta expresses SDH, particularly during the early stages of foetal development [10]. The specific activity of SDH in extracts prepared from the placenta at d19 of gestation was 0.34 ± 0.05 nmol/min/mg protein, or approximately 26% of the specific activity found in the maternal liver at the same stage of gestation.…”