Glutamine-dependent carbamyl phosphate synthetase during fetal and neonatal life in the rat☆

摘要

Thepyrimidine-synthesizingenzyme,carbamylphosphatesynthetaseII(CPsynthetaseII)wasexaminedintheratduringnormalfetaldevelopmentandinthefedandcaloricallydeprivedneonate.CPsynthetaseIIintheplacenta,liver,gut,carcass,andbrainshowedthefollowingcommonproperties;abilitytoutilizeammoniaaswellasl-glutamineasasubstrate;negligibleenhancementofactivitybyN-acetyll-glutamate;inhibitionofactivitybytheglutamineanalog,6-diazo-5-oxo-l-norleucine;andbythephosphorylatedpyrimidineuridine5a€2-triphosphate.ApparentKmvaluesforl-glutamineofCPsynthetaseIIinplacentaandextrahepaticfetalstructureswerefoundtovaryfrom1.1to2.3?—10a?’5M.Inthebrainandplacenta,tissueconcentrationsofl-glutamineobtainedatserialtimepointsduringgestationwereatleast200-foldhigher.Relativeactivitiesfortheenzymescatalyzingthesubsequenttwostepsinpyrimidinebiosynthesis,aspartatetranscarbamylaseanddihydroorotase,weresubstantiallygreaterthanCPsynthetaseIIatalltimesmeasuredandthereforewereconsistentwiththepossibilitythatCPsynthetaseIImaybeoneoftherate-limitingstepsinthedenovobiosynthesisofpyrimidinesintheplacentaandextrahepaticfetaltissues.Serialobservationswereobtainedinplacenta,brain,andneonatalmuscletoseewhethercorrelationscouldbedemonstratedbetweenconcentrationsofCPsynthetaseIIpermilligramoftissueDNAanddailyincrementsintotaltissueDNA.Inallthesestructures,higherconcentrationsofenzymewereobservedduringperiodsofmorerapidDNAaccumulation.Certainexceptionswerealsodemonstrable.Thus,manifestCPsynthetaseIIactivitypersistedintheplacentabeyondday16ofgestation(whenplacentalDNAnolongerincreases);andneonatalmuscleexhibitedCPsynthetaseIIactivitywhenallnetincrementsinDNAwereabolishedbycaloricdeprivation.Thelatterobservationshavesuggestedthattheenzymemaybeoperative(andofpossibleregulatorysignificance)evenintheabsenceofcellularproliferation.