HMBS activity

Additional tests ALAD activity l HMBS activity 1 Isomer separation Isocoproporphyrin Plasma scan ... 

If the patient is actually asymptomatic, but has a family history of acute porphyria or prior symptoms suspicious of acute porphyria, hydroxymethylbilane synthase (HMBS) activity, plasma scanning, and fecal porphyrins should be measured. These tests will reveal AIP, P V, and HC. As a small percentage of AIP families exhibit normal HMBS activity, PBG in a urine sample can be added. PBG determination can also performed as a first choice, if an acute porphyria is suspected. But if normal, it does not exclude acute porphyrias in asymptomatic phases. Furthermore, the existence of an acute porphyria is only proved if the value exceeds at least five times the upper limit of normal. 

Blood samples anticoagulated with heparin or EDTA may be used. We normally use washed erythrocytes (see below 7.3.3.7 HMBS Activity). 

E1MBS activity is lowered in AIP. In the case of a newly diagnosed acute porphyria, determination of HMBS activity enables the diagnosis of AIP. We further use it in AIP family screening combined with mutation analysis. 

We use a portionized sample from a volunteer with low normal HMBS activity. Analytical... 

HMBS activity 75-150 pmol/(mg hemoglobin h) (mean 2SD). Each laboratory should establish its own reference values. Those given can only be interpreted as a guide. 

As a heterozygous defect, HMBS activity is 50% of normal. In our experience symptomatic and asymptomatic patients with AIP show values in the range of 35-55 pmol/(mg hemoglobin h). There may be some overlap between low normal and high porphyric values. 

HMBS activity is usually below the reference interval in AIP, except in the uncommon nonerytliroid form but there is overlap between activities in AIP and normal individuals.After exclusion of families with nonerythroid AIP, HMBS activity has been reported to have a sensitivity of 84% and a specificity of 77% for detection of AIP in patients without symptoms. In France the prevalence of abnormally low HMBS activities in the general population is about 1 in 800. ... 

HMBS activity falls markedly as red cells age, so that any shift in their age distribution will be reflected in measured... 

CRY and PER proteins not only repress the activity of their own genes, but they also stimulate the expression of BMALl and CLOCK. How can the CRY and PER repressors activate the transcription of these positive hmb components The most likely scenario would imply a hitherto unknown repressor whose gene is under the negative control of CRYs and PERs. 

The thiolato complex [Ru(r 6-hmb)(en)SR]+ (hmb = hexamethylbenzene, R = Pr) can also undergo activation by ligand-based redox reactions involving a remarkably efficient oxygen-GSH couple under physiological conditions [115]. In these reactions GSH is almost completely oxidised to form GSSG and the thiolato complex is oxidised to the sulfenato complex [Ru(r 6-hmb)(en)S(0)R]+. 

Screening family members to identify asymptomatic individuals who have inherited AIP, VP, or HCP, and are therefore at risk for acute attacks, is an essential part of management of families with these disorders. Screening may be carried out by metabolite measurement, enzyme assay, DNA analysis, or a combination of these approaches. The most sensitive metabolite assays for the presymptomatic diagnosis of each disorder are listed in Table 32-7. These tests are almost always normal before puberty and therefore are not suitable for the investigation of children. In addition, urinary PEG excretion in AIP and the plasma fluorescence scan in VP may often be normal in asymptomatic adults shown by DNA analysis to be affected (Table 32-7). Measurement of the activity of the defective enzyme is more sensitive, but both sensitivity and specificity are limited by the overlap between activities in disease and in the normal population. Erythrocyte HMBS assay is widely used for the presymptomatic diagnosis of AIP. ... 

Fig. 2. Close up of the active site of HMBS with the first substrate molecule (PBG) in its proposed non-covalently bound orientation...

Uro gen III synthases from most sources are particularly unstable and can be totally inactivated by a brief heat-treatment which leaves HMBS fully active. This instability would frustrate any attempts to crystallise the enzyme. However, it has recently been reported that the UROS from B. subtilis is much more stable than from other sources [75] and this opens up the possibility of crystallisation and an X-ray crystal structure determination. 

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