Red Cell Enzyme Markers in Indian Populations: ADA, AK, ACP, PGM and Beyond

Blood groups usually steal the spotlight when people talk about human genetic diversity, but a quieter set of markers has done just as much work for anthropologists studying Indian populations: the enzymes and proteins carried inside red blood cells. These molecules keep cells running day to day, yet their inherited variants line up neatly with old migration routes, marriage circles and even historical malaria pressure across the subcontinent. This post walks through the classic red cell enzyme and protein systems studied in India, ADA, AK, ACP, PGM, 6-PGD, ESD, G6PD and haemoglobin, and unpacks what their frequency patterns reveal about who lives where, and why.

Why enzyme markers still matter in population genetics 🔗

Long before whole-genome sequencing became routine, researchers relied on biochemical markers to compare populations. These proteins are expressed co-dominantly, meaning both inherited forms show up on an electrophoresis gel, which makes it straightforward to calculate allele frequencies directly from blood samples. Once those frequencies are known for enough loci, researchers can estimate heterozygosity within a group and genetic distance between groups, the same basic statistics that later got applied to DNA markers. Bodies like the Anthropological Survey of India used exactly this approach for decades to map genetic distances between castes, tribes and linguistic groups.

These systems are often called classical markers, to distinguish them from the molecular markers, STRs, SNPs and full genome sequences, that came later. The result is a rich dataset of enzyme and protein frequencies that, even today, complements DNA-based studies rather than being replaced by them. Several of the broad population divisions first noticed through these enzyme systems have since been confirmed, and refined, by genomic research.

Adenosine deaminase (ADA): a purine-metabolism enzyme with regional stories 🔗

ADA is an aminohydrolase that deaminates adenosine into inosine, a step in purine breakdown. Three phenotypes appear on gels: ADA1, ADA2 and ADA2-1, controlled by two co-dominant alleles, ADA*1 and ADA*2. Across India, ADA*1 is by far the more common allele, averaging around 0.88 and ranging from 0.50 to 0.98 depending on the population studied.

The same gene has a separate claim to fame in clinical genetics, since certain rare mutations at this locus cause a form of severe combined immunodeficiency, but the common ADA*1/ADA*2 variation discussed here is unrelated to that disease and simply reflects normal population variation. Western, north-eastern and southern Indian groups tend to sit at the higher end of the frequency range for ADA*1. Among tribal groups of Andhra Pradesh, including the Jalari, Lambada and Savara, as well as Hindu caste populations of the same region, ADA shows clear polymorphism alongside AK1, ACP, PGM1, 6-PGD and G6PD, confirming that this enzyme varies meaningfully even within a single state.

Adenylate kinase (AK): a marker of Himalayan and Mongoloid ancestry 🔗

AK catalyses the interconversion of the three adenine nucleotides, ATP, ADP and AMP, a reaction central to cellular energy balance. Three phenotypes are recognised, AK1, AK2 and AK3, arising from the alleles AK*1, AK*2 and AK*3. The AK*1 allele dominates nationally, averaging 0.92 and ranging from 0.79 to as high as 1.00 in some groups.

Populations of the Himalayan mountain belt, groups with Mongoloid affinity in the eastern Himalayas, and Tibeto-Burman speakers consistently show the highest AK*1 frequencies. Scheduled Tribes of the southern and western peninsula, by contrast, show comparatively lower values, a pattern that recurs across several of the markers discussed below and hints at long-standing geographic separation between these regions.

Acid phosphatase (ACP): caste, language and a rare Himalayan allele 🔗

ACP is a phosphohydrolase that breaks down phosphate esters under acidic conditions. It produces four recognisable phenotypes, ACP1A, ACP1BA, ACP1CA and ACP1CB, governed by three alleles: ACP1*A (0.10 to 0.36), ACP1*B (0.64 to 0.86) and a rare ACP1*C (0.00 to 0.0035).

ACP1*A reaches its highest frequencies among Scheduled Castes of the Indus-Ganga-Brahmaputra plain and among Indo-European language speakers more broadly. The rare ACP1*C allele is notable mainly for where it is absent: it turns up almost exclusively in Himalayan populations and is essentially undetectable elsewhere in the country. Across tribal groups of central India, too, ACP was one of several enzyme systems found to be genuinely polymorphic, with variation staying broadly within the range recorded for the rest of the country, unlike monomorphic systems such as LDH or GPI that show almost no variation anywhere.

Phosphoglucomutase and 6-phosphogluconate dehydrogenase: two glucose-processing enzymes 🔗

PGM: shuttling sugar for glycogen metabolism 🔗

Phosphoglucomutase interconverts glucose-6-phosphate and glucose-1-phosphate, a step needed for glycogen synthesis and breakdown. The common allele, PGM1*1, averages 0.70 nationally but ranges widely, from 0.44 to 0.95. Scheduled Castes of the southern peninsula, Tibeto-Burman speakers, and populations of West Bengal and Karnataka tend to show the higher end of this spectrum.

6-PGD: an enzyme of the pentose phosphate pathway 🔗

6-Phosphogluconate dehydrogenase oxidatively decarboxylates 6-phosphogluconate into ribulose-5-phosphate, generating NADPH along the way, a reaction step in the pentose phosphate pathway that helps protect the cell from oxidative damage. Its common allele, PGD*A, averages 0.95 and ranges from 0.75 to 1.00. Scheduled Caste populations and groups with Mongoloid affinity show the highest PGD*A frequencies recorded in Indian samples.

Esterase D and glucose-6-phosphate dehydrogenase: detox and redox enzymes 🔗

Esterase D: quietly detoxifying formaldehyde 🔗

ESD is a serine hydrolase involved in breaking down formaldehyde inside cells. Its common allele, ESD*1, averages 0.72 and ranges from 0.41 to 0.97 across studied groups. Himalayan populations and Indo-European speakers show the highest frequencies of this allele, echoing the regional pattern already seen for AK and ADA.

G6PD deficiency: where enzyme genetics meets malaria history 🔗

G6PD catalyses a redox reaction that regenerates NADPH, which red blood cells rely on to withstand oxidative stress. Deficiency in this enzyme averages around 4.5 per cent nationally but ranges from 0 to 27 per cent, and this is not random. Deficiency frequencies climb highest in regions where malaria is, or historically was, endemic, since a partially reduced enzyme activity offers some protection against the malaria parasite while it is developing inside red blood cells.

Himalayan populations and groups with Mongoloid affinity report the highest deficiency rates in the country, while Dravidian-speaking groups and populations of Andhra Pradesh tend to run lower. The genetic detail behind this is telling: a specific mutation known as G6PD Namoru appears almost exclusively among Dravidian-speaking tribes of the Nilgiri hills, while other mutations dominate in different linguistic zones, showing that G6PD deficiency is really a patchwork of distinct mutations rather than one uniform variant spreading evenly across the country. This distinction has practical weight even today: health programmes in malaria-endemic tribal districts now recommend screening for G6PD deficiency before prescribing primaquine, the drug used against relapsing malaria, precisely because deficient individuals can develop severe anaemia on standard doses.

Haemoglobin variants: HbS and HbE trace two different histories 🔗

Haemoglobin is a tetramer built from two alpha and two beta globin chains, and it is the molecule responsible for carrying oxygen, carbon dioxide and nitric oxide through the bloodstream. Two structural variants stand out in Indian population genetics for very different geographic reasons.

HbS: a marker of central India’s tribal and scheduled caste belt 🔗

HbS averages 0.031 nationally but ranges from 0 to as high as 0.41 in some Scheduled Tribe and Scheduled Caste groups of the semi-arid steppe and tropical savannah zones of central India. Tribal populations of central India and eastern Odisha and Jharkhand show HbS as their predominant haemoglobin disorder, a pattern long linked to historical exposure to falciparum malaria in these regions.

The social gradient here is striking. A large study from eastern Maharashtra found HbS ranging from 0 to 24 per cent among Scheduled Tribes, 0 to 13 per cent among Scheduled Castes and nomadic groups, 0 to 20 per cent among Other Backward Classes, and only 0 to 5 per cent in higher caste populations, though the gene has also been detected outside these traditionally studied groups.

HbE: a signature of the eastern Himalayas 🔗

HbE tells a completely different geographic story. It averages 0.023 nationally but can reach 0.64 in specific communities, concentrated almost entirely among Mongoloid-affinity populations of the eastern Himalayas, along with Mon-Khmer Austro-Asiatic and Tibeto-Burman speakers of India’s north-east. This regional concentration lines up with the broader pattern seen for AK, ESD and other markers discussed above, where north-eastern populations consistently stand apart from the rest of the subcontinent.

What these frequency patterns add up to 🔗

Put side by side, these enzyme and haemoglobin systems tell a consistent story. Himalayan and north-eastern populations with Mongoloid affinity repeatedly cluster together across unrelated loci, AK, ESD, G6PD deficiency and HbE all point the same way. Peninsular Scheduled Tribes and Scheduled Castes form a separate cluster, most visible in HbS and PGM1 frequencies. Meanwhile, enzymes tied to oxidative stress protection, G6PD and, indirectly, the haemoglobin variants, track old malaria zones rather than simple caste hierarchy or geography alone. That is a case of natural selection, not just drift or endogamy, shaping the gene pool.

None of this required DNA sequencing to discover. Researchers built these pictures using gel electrophoresis and careful sampling across decades, and the broad population divisions they identified have largely held up as newer genomic tools have come along. Language family also turns out to matter almost as much as geography: Indo-European speakers, Dravidian speakers, Tibeto-Burman speakers and Austro-Asiatic speaking groups each show a recognisable signature across these enzyme systems, often more consistently than caste category alone. That continuity is itself a useful lesson about how classical markers and modern genomics complement rather than compete with each other, and about why a subject as old as biochemical genetics still has a place in a modern anthropology classroom.

What do you think? Classical enzyme markers like ADA and PGM were mapped out decades before affordable whole-genome sequencing arrived in India. Do studies like these still add real value alongside modern DNA-based population genetics, or have they been fully overtaken? And given how closely G6PD deficiency and HbS track historical malaria zones, what does that suggest about how much an infectious disease can reshape a population’s genetics over generations?

References

1. https://pubmed.ncbi.nlm.nih.gov/8845341/
2. https://www.nature.com/articles/jhg199323
3. https://www.sciencedirect.com/science/article/abs/pii/S1567134820304287
4. https://pubmed.ncbi.nlm.nih.gov/26139767/
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9670505/
6. https://ijmr.org.in/haemoglobinopathies-in-tribal-populations-of-india/
7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3708271/