Chemical Patholgy 1 - Uric acid metabolism Flashcards

1
Q

Recall 3 roles of purines

A
  1. Base in DNA
  2. 2nd messengers for cAMP
  3. Form part of ATP
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2
Q

Recall the pathway of purine catabolism

A

Purines –> hypo-xanthine –> xanthine –> uric acid

Xanthine oxidase is involved in the production of both xanthine and urate

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3
Q

Recall the renal handling of urate

A

Urate is fairly insoluble
It is freely-filtered at the Bowman’s capsule
Bizzarely: it is both reabsorbed and re-excreted in the proximal convoluted tubule!

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4
Q

Name one disorder of inborn error of purine

metabolism

A

Lesch Nyhan Syndrome

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5
Q

What is the inheritance pattern of Lesch Nyhan syndrome

A

X-linked

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6
Q

How does Lesch-Nyhan syndrome first present?

A

With developmental delay at 6-12 months

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7
Q

What is the mutation that causes Lesch-Nyhan syndrome?

A

HPRT - an enzyme that is key in purine recycling

Since there is no feedback inhibition on dinovosynthesis of purines, plasma urate increases a LOT

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8
Q

What are the symptoms of Lesch-Nyhan syndrome?

A
Choreform movements (rythmic almost dancelike spasms/movements), spasticity and UMN signs with mental retardation
Self-mutilation in 85% :-( They bite their lips and digits with a great deal of force

HPRT mutation, leading to no salvage pathway, reduced negative inhibition on PAT and thus hyperurea

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9
Q

What type of crystals cause gout?

A

Monosodium urate

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10
Q

What are the 2 clinical forms of gout?

A

Acute - “podagra”

Chronic - “tophaceous”

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11
Q

How should acute gout be treated?

A

Key thing is to reduce inflammation
1. NSAIDs
1b. Colchicine (this is 2nd line, if NSAIDs contra-indicated)
1c. Glucocorticoids (if all else fails!!)
Nb: if you try to correct urate in the acute phase, you can actually make it worse!!

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12
Q

How should chronic gout be treated?

A

Need to manage the hyperuricaemia

  1. Hydration (water!)
  2. Reverse the factors driving urate up
  3. Allopurinol
  4. Probenecid
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13
Q

What prescription drug can drive urate up?

A

Thiazide diuretics

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14
Q

What is the mechanism of action of allopurinol?

A

Inhibits xanthine oxidase

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15
Q

What is the mechanism of action of probenecid?

A

Increases renal excretion of urate

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16
Q

Recall one very important drug interaction to avoid when prescribing gout drugs

A

Allopurinol and azothioprine

The intermediary of azothioprine = mercaptopurine - which needs xanthine oxidase to be catabolised

17
Q

Recall the diagnostic approach for gout

A
  1. History and examination should be sufficient

2. If not clear: TAP effusion

18
Q

How can gout crystrals be visualised?

A

View effusion material under polarised light - use RED filter

19
Q

How can gout and pseudogout crystals be differenitated under the microscope?

A

Gout crystals = negatively birefringent (BLUE - this is perpendicular to compensator, orange colour will be parallel)
Pseudogout crystals - positively birefringent (red)

20
Q

Which other condition predisposes to pseudogout?

A

Osteoarthritis

21
Q

How long does pseudogout last?

A

1-3 weeks