Question 1

Which of the following characteristics is true of hemosiderin that differentiates it from ferritin?

Accepted Answer

Hemosiderin is an insoluble complex derived from ferritin that has lost part of its surface protein.The function of the surface protein is to maintain complex solubility.Even though hemosiderin has a higher iron concentration than ferritin, the insolubility of hemosiderin causes a delay in release of iron from the complex.

Question 2

Porphyrias are characterized by reduced activity or a deficiency of a specific enzyme.If methods were available to accurately measure activity of each enzyme in the heme synthetic pathway, why would the red blood cell not be a good place to quantify enzyme activity?

Accepted Answer

The synthetic pathway for heme synthesis begins and ends in the mitochondrion of the cell.The middle steps in this pathway occur in the cytoplasm.Given that developing red blood cells lose their mitochondria as they mature, only half of the enzymes of the heme synthetic pathway would be available for analysis in circulating red blood cells.

Question 3

In variegate hepatic porphyria, which of the following would show the greatest increase?

Accepted Answer

Variegate porphyria is associated with a partial deficiency of protoporphyrinogen oxidase, which is responsible for converting protoporphyrinogen to protoporphyrin in the heme synthetic pathway.Thus there is a buildup of protoporphyrinogen.Protoporphyrinogen, as with all porphyrinogens, is spontaneously and irreversibly oxidized to its corresponding porphyrin, protoporphyrin.Protoporphyrin, now outside of the mitochondrion, is unable to participate in heme synthesis and is excreted at increased levels in the feces.

Question 4

______ is the porphyrin most likely to be exclusively excreted in urine, and ______ is the porphyrin most likely to be exclusively excreted in feces.______ has the greatest chance of appearing in either urine or feces.

Accepted Answer

Uroporphyrin contains eight carboxyl groups, coproporphyrin contains four carboxyl groups, and protoporphyrin contains two carboxyl groups.Uroporphyrin is the most polar of the three porphyrins, making it the most water soluble and the most likely to appear in urine.Protoporphyrin is the least polar of the three porphyrins and will likely be excreted only in feces.Coproporphyrin is intermediary in its polarity, allowing this porphyrin to appear in both urine and feces.

Question 5

Which of the following is/are true of hepcidin?

Accepted Answer

Hepcidin is a protein hormone produced by the liver; it is important in the regulation of iron balance.High levels of hepcidin restrict the flow of iron out of intestinal cells by binding to ferroportin and inhibiting iron release.Low levels of hepcidin do not interfere with the flow of iron from the intestine to the circulation.

Question 6

At present, the one heme enzyme that is routinely measured in clinical laboratories is:

Accepted Answer

Measurements of the heme enzymes have shown limited application in the clinical laboratory because of problems such as technical difficulties with assays and the fact that not all heme enzymes may be available in the mature red blood cell.One enzyme that is measured regularly in clinical laboratories is porphobilinogen deaminase.Porphobilinogen deaminase activity is decreased in patients with acute intermittent porphyria.

Question 7

In which of the following neurological porphyrins is a parallel increase in δ-aminolevulinic (ALA) acid and porphobilinogen (PBG) not seen?

Accepted Answer

Porphyria cutanea tarda does not show an increase in ALA and PBG.The enzyme that is deficient in this porphyria is uroporphyrinogen decarboxylase, which would lead to an increase of urinary uroporphyrin but not urinary ALA and PBG.

Question 8

Iron in the Fe²⁺ state is known as ______ iron, and iron in the Fe³⁺ state is known as ______ iron.The conversion of Fe³⁺ to Fe²⁺ is known as ______.

Accepted Answer

Fe²⁺ is known as ferrous iron.Fe³⁺ is known as ferric iron.Most dietary iron is in the ferric state and must be converted to the ferrous state before it can enter the intestinal cell.Ferric reductase is the enzyme responsible for the reduction of ferric iron to ferrous iron.

Question 9

In hepatic porphyria cutanea tarda, which of the following will show the greatest increase?

Accepted Answer

Hepatic porphyria cutanea tarda is caused by a partial deficiency of uroporphyrinogen decarboxylase.As a result of this deficiency, there will be an accumulation of uroporphyrinogen, which will readily oxidize to uroporphyrin.Uroporphyrin, being relatively polar, will be excreted into the urine.

Question 10

Percent saturation is a useful expression that relates:

Accepted Answer

Percent saturation is calculated by: (serum iron/TIBC) × 100%.TIBC serves as an indirect measure of transferrin, and the percent saturation calculation serves as an indicator of the degree of iron saturation of transferrin.

Question 11

Free erythrocyte protoporphyrin is somewhat of a misnomer.This form of protoporphyrin that is found elevated in the erythrocyte in certain disorders is actually bound to ________ instead of being free in the erythrocyte.

Accepted Answer

During periods of iron deficiency, zinc becomes the alternative metal for ferrochelatase to insert into the free protoporphyrin molecule, forming zinc protoporphyrin.Thus free erythrocyte protoporphyrin is also known as zinc protoporphyrin.

Question 12

Data below provide laboratory evidence of which pathological condition?
Red cell morphology - hypochromia, microcytosis
Serum iron - increased
TIBC - normal
Percent saturation - increased
Serum ferritin - increased

Accepted Answer

Sideroblastic anemia is a disorder in which the body has adequate stores of iron but is unable to incorporate this iron into protoporphyrin to form hemoglobin.Red cells poorly develop, resulting in a hypochromic, microcytic appearance under microscopic examination.Because of the inability to utilize iron properly, serum iron levels, percent saturation, and ferritin levels will be increased, and TIBC levels will be normal.

Question 13

Which of the following would be expected in association with kidney disease?

Accepted Answer

Iron balance in the body occurs primarily by the regulation of the absorption of iron rather than by excretion of excess iron.Changes in kidney function should not have a significant impact on iron levels.The major factors that regulate iron absorption are body iron stores and red cell production.The hormone hepcidin is responsible for controlling iron absorption.

Question 14

It is known that porphyrins are colored compounds, whereas porphyrinogens are colorless compounds.What is the chemical difference between porphyrins and porphyrinogens that accounts for this difference?

Accepted Answer

Porphyrins have a macrocyclic ring structure that is completely conjugated (i.e., alternating double and single bonds), meaning that these compounds are highly unsaturated.This conjugated (or unsaturated) nature of the molecules results in these molecules being highly colored.When porphyrins are reduced to porphyrinogens by the addition of protons across some of the double bonds, the degree of saturation is increased, causing these compounds to become colorless.

Question 15

Which of the following is considered the most appropriate method for quantifying porphyrins in blood or urine?

Accepted Answer

Porphyrins are cyclic molecules consisting of alternating single and double bonds.This extended conjugation pattern imparts a significant fluorescent nature to porphyrins.Given that fluorescence measurements are approximately 1000 times more sensitive than spectrophotometric measurements, and that porphyrins are fluorescent by nature, fluorometry is the most appropriate method for quantifying porphyrins in blood or urine.

Question 16

The following data provide laboratory evidence of which disorder of porphyrin metabolism?
Urinary δ-aminolevulinic acid (ALA) - elevated
Urinary porphobilinogen (PBG) - normal
Free erythrocyte protoporphyrin (zinc protoporphyrin) - elevated

Accepted Answer

The laboratory findings of elevated urinary δ-aminolevulinic acid (ALA), normal urinary porphobilinogen (PBG), and elevated free erythrocyte protoporphyrin (zinc protoporphyrin) are indicative of lead poisoning. Lead inhibits several enzymes in the heme synthesis pathway, leading to the accumulation of precursors such as ALA and zinc protoporphyrin.

Question 17

The rate-limiting step in heme synthesis in the liver is regulated by which enzyme?

Accepted Answer

Heme synthesis in the liver is controlled by ALA synthase activity.ALA synthase is regulated by the amount of free heme available within liver cells.In the presence of excess heme, ALA synthase activity is decreased, and it is stimulated by a decrease in free heme.

Question 18

Data below provide laboratory evidence of which pathological condition?
Red cell morphology - hypochromia, microcytosis
Serum iron - decreased
TIBC - increased
Percent saturation - decreased
Serum ferritin - decreased

Accepted Answer

Iron deficiency anemia occurs when there is not enough iron in the body to meet its needs.Since there is not enough iron to incorporate into hemoglobin, red cells will appear as hypochromic and microcytic.Serum iron, percent saturation, and serum ferritin levels will be decreased due to lack of available iron.TIBC will be increased due to increased availability for iron binding sites on transferrin.

Quiz 39: Iron and Porphyrin Metabolism