Clinical Biochem

 

CLINICAL QUESTIONS 81. Oral contraceptives are known to lower blood levels of folate and vitamin B6. Which of the following conversion processes might be directly affected? A. Ornithine to Citrlline B. !istidine to histamine C. "ltamate to "ltamine D. #ropionyl co $ to %ethyl malonyl co $ E. #henyl alanine to &yrosine $nswer' B' &he correct answer is conversion of !istidine to histamine. &he reaction is cataly(ed by decarbo)ylase that re*ires B6'# as a cofactor +,igre'1- that gets deficient in women pon long'term oral contraceptive sage. ,olic acid is not re*ired in this reaction.

,igre'1' ecarbo)ylation of !istidine is carried ot by histidine decarbo)ylase. !istamine is an important chemical for allergies and anaphyla)is. #/# is pyrido)al phosphate+B6'#&he conversion of Ornithine to citrlline +,igre'0- takes place in rea formation. for mation. one of these vitamin B6 or folic acid is re*ired in this conversion process.

,igre'0' Ornithine to citrlline conversion is carried ot by ornithine transcarbamoylase2 which is an important mitochondrial en(yme re*ired for rea synthesis. "ltamate to gltamine conversion is cataly(ed by gltamine synthetase +figre'3-. 4t is a reaction to deto)ify ammonia2 the first line of defense in brain. &hese vitamins are not re*ired in this reaction process.

 

,igre'3' "ammagltamyl phosphate is an intermediate in the conversion of "ltamate to gltamine2 #ropionyl co$ to %ethylmalonyl co$ conversion re*ires biotin as a coen(yme +figre'5-. 4t is a carbo)ylation reaction $& $&# is also re*ired for this process.

,igre'5' #ropionyl co$ to methylmalonyl co$ conversion conversion is an intermediate step in the actal conversion of propionyl co$ to 7ccinyl co$2 which is an intermediate of &C$ cycle. #henylalanine to &yrosine &yrosine conversion re*ires &e &etrahydrobiopterin trahydrobiopterin +B!5- as a coen(yme. &he reaction is cataly(ed by #henylalanine hydro)ylase and it is the first step of metabolism of  phenylalanine +figre'+figre'-

,igre'' #henylalanine hydro)ylase cataly(ing the conversion of phenylalanine to tyrosine is a monoo)ygenase2 deficiency of which cases #henylketonria.

 

80. ring a medical rotation2 a medical stdent volnteered for a respiratory r espiratory physiology e)am that determines basal metabolic rate and the respiratory *otient. 7he followed the protocol for a resting individal in the post absorptive state. Which of the following amino acids wold be fond in the highest concentration in serm? A. B. C. D. E.

$lanine and "ltamine $rginine and Ornithine "ltamate and $spartate Branched chain amino acids !ydrophobic amino acids

$nswer' $' $lanine and "ltamine. #ost absorptive state is the state of the body 3' hors after meals. 4t is that period of time in the body dring which cell metabolism is primarily feled by ntrients stored from previos meals this state is dominated by glcagon release from the alpha cells of the pancreatic islets in response to hypoglycemia varios 9inslin antagonists2: antagonists2: thyroid hormones +&3 and &5-2 glcocorticoids +cortisone2 cortisol2 hydrocortisone- from the adrenal corte)2 epinephrine from the adrenal medlla2 and hman growth hormone from the anterior  pititary are also present present in increased amonts and and and contribte to the physiological processes processes which occr in this state. #rimarily2 hepatocytes hepatocytes are the responsive r esponsive targets to glcagon and2 in response r esponse to glcagon arrival2 release glcose into the plasma to be sed by nervos tisse in o)idative pathways to generate $&#. &he liver obtains this glcose in two ways; +1-

by rele releasi asing ng glc glcose ose whi which ch had had been been sto stored red as as glyco glycogen gen +gl +glyco ycogen genoly olysis sis-- and and

+0- by synth synthesi esi(in (ing g new glc glcose ose mole molecl cles es from from the brea breakdo kdown wn prod prodcts cts of of lipid lipid and and protei protein n catabolism +glconeogenesis%ost other tisses shift to energy prodction from lipid and protein catabolism. $dipocytes $dipocytes liberate fats +triglycerides- into the bloodstream +lipolysis- to spport the metabolism of these other tisses2 the glycerol released from triglycerides is sed to synthesi(e glcose +glconeogenesis- the liver o)idi(es free fatty acids for fel2 prodcing ketone bodies +ketogenesis- as by'prodcts +figre ervos tisse is not responsive responsive to glcagon2 bt contines to tili(e tili(e the glcose.

 

,igre' Changes occrring in the post absorptive state 4n the post absorptive state2 the mscles release alanine and gltamine which are fond in highest concentration. %ost of the alanine is removed by liver +glcose' alanine cycle- and gltamine is taken p by kidney. &he carbon skeleton of alanine is sed for glcose prodction whereas amino grop is removed in the form for m of ammonia which is deto)ified forming rea. 4n the kidney gltamine is hydroly(ed by gltaminase en(yme forming ammonia and gltamate. $mmonia is converted to ammonim ion and contribtes to acid secretion by the kidney.